| 0/0 |
2026/05/15 10:32 |
flow |
assessment-security |
1h23m
Results: map[DenialOfService:false Explanation:Based on the analysis of the stack traces and the kernel source code, here is
the assessment of the bug:
**Root Cause Analysis:**
The bug report indicates an RCU stall where tasks are blocked on an RCU node,
preventing the RCU grace period kthread from completing its work. The stack
traces for the blocked tasks (`syz-executor`) show that they are in the process
of freeing a large number of pages during file eviction/truncation
(`shmem_undo_range` and `truncate_inode_pages_range`).
Specifically, the tasks are deep inside the `page_owner` tracking code
(`__reset_page_owner`, `save_stack`, `unwind_next_frame`, and
`__update_page_owner_free_handle`). `page_owner` is a kernel debugging feature
that records the allocation and freeing stack traces for every single page in
the system. Stack unwinding (`unwind_next_frame`) is a computationally expensive
and slow operation.
When a process deletes or truncates a very large file, it frees thousands or
millions of pages in a loop. With `page_owner` enabled, the kernel has to unwind
and save the stack trace for every single page being freed. This massive
overhead keeps the CPUs busy for an extended period (over 10.5 seconds in this
report), which starves the `rcu_preempt` kthread and triggers the RCU stall
detector.
**Security Impact:**
The `page_owner` feature is strictly a debugging and memory profiling tool
(enabled via `CONFIG_PAGE_OWNER=y` and the `page_owner=on` boot parameter). It
is **not enabled in production kernel builds** due to its severe performance
overhead.
In a standard production environment without `page_owner` and other heavy
debugging tools (like KASAN), freeing pages is an extremely fast operation. The
loops in `shmem_undo_range` and `truncate_inode_pages_range` will complete
quickly and will not cause an RCU stall or CPU lockup. There are no infinite
loops in these functions, as the page cache iterators correctly advance their
indices regardless of whether the pages are immediately freed.
Therefore, this is a false positive caused by the artificial overhead of
debugging tools. It does not represent a Denial of Service (DoS) or an
exploitable vulnerability in production systems.
**Trigger Vectors:**
While the bug itself is not present in production, the underlying code path
(creating and deleting/truncating large files) is reachable by any unprivileged
user, including those inside user namespaces or running within a VM guest. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:true VMHostTrigger:false]
|
| 1/1 |
2026/05/15 10:32 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/05/15 10:32 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/049da299ef819359e093c7f86424c4dc2136bed6]
|
| 3/1 |
2026/05/15 10:32 |
action |
kernel-builder |
29m
Results: map[KernelObj:/app/workdir/cache/build/f3f068fea37802b9ad8aaefd3c340106112c5ff3]
|
| 4/1 |
2026/05/15 11:02 |
action |
codesearch-prepare |
21m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/05/15 11:23 |
agent |
expert |
32m
Model:
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:true VMHostTrigger:false]
Instruction:
You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by greeping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Don't make assumptions; verify them with source code access. Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption or elevated privileges.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completly wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control tranfers, that's also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitabability depends on how exactly the uninit value
is used in the code, and what it affects.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidetial Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The kernel bug report is:
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: Tasks blocked on level-0 rcu_node (CPUs 0-1): P25104/1:b..l P28178/1:b..l
rcu: (detected by 0, t=10502 jiffies, g=203761, q=580 ncpus=2)
task:syz-executor state:R running task stack:22216 pid:28178 tgid:28178 ppid:28167 task_flags:0x400140 flags:0x00080001
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5388 [inline]
__schedule+0x1821/0x5740 kernel/sched/core.c:7189
preempt_schedule_irq+0x4d/0xa0 kernel/sched/core.c:7513
irqentry_exit_to_kernel_mode include/linux/irq-entry-common.h:547 [inline]
irqentry_exit+0x14f/0x760 kernel/entry/common.c:164
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:unwind_next_frame+0x19f0/0x2550 arch/x86/kernel/unwind_orc.c:693
Code: 49 8d 56 40 4c 89 f7 4c 89 e6 e8 1b 0e 00 00 84 c0 0f 84 46 01 00 00 48 bd 00 00 00 00 00 fc ff df 48 8b 44 24 20 0f b6 04 28 <84> c0 0f 85 6a 08 00 00 b3 01 8b 84 24 8c 00 00 00 41 39 06 4c 8b
RSP: 0018:ffffc9000561f118 EFLAGS: 00000202
RAX: 0000000000000000 RBX: ffffffff90d3be2e RCX: 0000000000000001
RDX: ffffc9000561f228 RSI: dffffc0000000000 RDI: ffffc9000561f448
RBP: dffffc0000000000 R08: ffffc9000561f448 R09: ffffc9000561f238
R10: dffffc0000000000 R11: fffff52000ac3e49 R12: ffffc9000561f448
R13: 1ffff92000ac3e3f R14: ffffc9000561f1e8 R15: 1ffffffff21a77c6
arch_stack_walk+0x11b/0x150 arch/x86/kernel/stacktrace.c:25
stack_trace_save+0xa9/0x100 kernel/stacktrace.c:122
save_stack+0x122/0x230 mm/page_owner.c:165
__reset_page_owner+0x71/0x1f0 mm/page_owner.c:320
reset_page_owner include/linux/page_owner.h:25 [inline]
__free_pages_prepare mm/page_alloc.c:1402 [inline]
free_unref_folios+0xcec/0x1480 mm/page_alloc.c:3004
folios_put_refs+0x9ff/0xb40 mm/swap.c:1008
folio_batch_release include/linux/folio_batch.h:101 [inline]
shmem_undo_range+0x52c/0x1660 mm/shmem.c:1149
shmem_truncate_range mm/shmem.c:1277 [inline]
shmem_evict_inode+0x289/0xae0 mm/shmem.c:1407
evict+0x61e/0xb10 fs/inode.c:841
__dentry_kill+0x1a2/0x690 fs/dcache.c:718
finish_dput+0xc9/0x480 fs/dcache.c:927
__fput+0x691/0xa60 fs/file_table.c:518
task_work_run+0x1d9/0x270 kernel/task_work.c:233
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
__exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
exit_to_user_mode_loop+0xf3/0x4d0 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:238 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x33e/0xf80 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f91bdd5d68e
RSP: 002b:00007fff641bc348 EFLAGS: 00000246 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 000055556c51b500 RCX: 00007f91bdd5d68e
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007fff641bc3ec R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000035
R13: 00000000000927c0 R14: 00000000001861a8 R15: 00007fff641bc440
</TASK>
task:syz-executor state:R running task stack:22360 pid:25104 tgid:25104 ppid:25089 task_flags:0x400140 flags:0x00080001
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5388 [inline]
__schedule+0x1821/0x5740 kernel/sched/core.c:7189
preempt_schedule_irq+0x4d/0xa0 kernel/sched/core.c:7513
irqentry_exit_to_kernel_mode include/linux/irq-entry-common.h:547 [inline]
irqentry_exit+0x14f/0x760 kernel/entry/common.c:164
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:__nr_to_section include/linux/mmzone.h:2061 [inline]
RIP: 0010:__pfn_to_section include/linux/mmzone.h:2198 [inline]
RIP: 0010:lookup_page_ext mm/page_ext.c:255 [inline]
RIP: 0010:page_ext_lookup+0x50/0x180 mm/page_ext.c:513
Code: ee 16 e9 2d 01 00 00 e8 9e 40 89 ff 41 bf 00 00 04 00 49 bd 00 00 00 00 00 fc ff df 4c 89 f7 4c 89 fe e8 e3 42 89 ff 4d 39 fe <0f> 83 ef 00 00 00 48 c7 c0 00 ba 4a 9a 48 c1 e8 03 42 80 3c 28 00
RSP: 0018:ffffc90009bbf348 EFLAGS: 00000287
RAX: ffffffff823c814d RBX: 00000000000a7513 RCX: ffff88804ea1bd80
RDX: 0000000000000000 RSI: 0000000000001000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffffffff823ac26e R09: ffffffff8e95cd60
R10: 000000000000000f R11: ffffffff81b0ebd0 R12: 00000000000a7513
R13: dffffc0000000000 R14: 0000000000000000 R15: 0000000000001000
page_ext_iter_begin include/linux/page_ext.h:133 [inline]
__update_page_owner_free_handle+0x103/0x470 mm/page_owner.c:284
__reset_page_owner+0x85/0x1f0 mm/page_owner.c:321
reset_page_owner include/linux/page_owner.h:25 [inline]
__free_pages_prepare mm/page_alloc.c:1402 [inline]
free_unref_folios+0xcec/0x1480 mm/page_alloc.c:3004
folios_put_refs+0x9ff/0xb40 mm/swap.c:1008
folio_batch_release include/linux/folio_batch.h:101 [inline]
truncate_inode_pages_range+0x3dd/0xe30 mm/truncate.c:408
ext4_evict_inode+0x2d9/0x10e0 fs/ext4/inode.c:197
evict+0x61e/0xb10 fs/inode.c:841
dispose_list fs/inode.c:883 [inline]
evict_inodes+0x75a/0x7f0 fs/inode.c:937
generic_shutdown_super+0xaa/0x2d0 fs/super.c:632
kill_block_super+0x44/0x90 fs/super.c:1725
ext4_kill_sb+0x68/0xb0 fs/ext4/super.c:7492
deactivate_locked_super+0xbc/0x130 fs/super.c:476
cleanup_mnt+0x437/0x4d0 fs/namespace.c:1312
task_work_run+0x1d9/0x270 kernel/task_work.c:233
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
__exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
exit_to_user_mode_loop+0xf3/0x4d0 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:238 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x33e/0xf80 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f1fc999e097
RSP: 002b:00007ffcfafecef8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
RAX: 0000000000000000 RBX: 00007f1fc9a321ca RCX: 00007f1fc999e097
RDX: 0000000000000000 RSI: 0000000000000009 RDI: 00007ffcfafecfb0
RBP: 00007ffcfafecfb0 R08: 00007ffcfafedfb0 R09: 00000000ffffffff
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffcfafee040
R13: 00007f1fc9a321ca R14: 0000000000186e02 R15: 00007ffcfafee080
</TASK>
rcu: rcu_preempt kthread starved for 10598 jiffies! g203761 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
rcu: RCU grace-period kthread stack dump:
task:rcu_preempt state:R running task stack:27592 pid:16 tgid:16 ppid:2 task_flags:0x208040 flags:0x00080000
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5388 [inline]
__schedule+0x1821/0x5740 kernel/sched/core.c:7189
__schedule_loop kernel/sched/core.c:7268 [inline]
schedule+0x164/0x360 kernel/sched/core.c:7283
schedule_timeout+0x158/0x2c0 kernel/time/sleep_timeout.c:99
rcu_gp_fqs_loop+0x312/0x11d0 kernel/rcu/tree.c:2095
rcu_gp_kthread+0x9e/0x2b0 kernel/rcu/tree.c:2297
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
rcu: Stack dump where RCU GP kthread last ran:
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
RIP: 0010:pv_native_safe_halt+0xf/0x20 arch/x86/kernel/paravirt.c:63
Code: 9b 7d 02 e9 fe 7f 67 f5 cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 66 90 0f 00 2d 53 a1 20 00 fb f4 <e9> d7 7f 67 f5 cc cc cc cc cc cc cc cc cc cc cc cc 90 90 90 90 90
RSP: 0018:ffffc90000197e20 EFLAGS: 00000242
RAX: 0000000002a0c245 RBX: ffffffff819a93ba RCX: 0000000080000001
RDX: 0000000000000001 RSI: ffffffff8dfa6837 RDI: ffffffff8c28af60
RBP: ffffc90000197f10 R08: ffff8880b87339db R09: 1ffff110170e673b
R10: dffffc0000000000 R11: ffffed10170e673c R12: 0000000000000001
R13: 1ffff11003b54000 R14: 0000000000000001 R15: 1ffff11003b54000
FS: 0000000000000000(0000) GS:ffff88812538d000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b3041bff8 CR3: 0000000096247000 CR4: 0000000000350ef0
Call Trace:
<TASK>
arch_safe_halt arch/x86/kernel/process.c:766 [inline]
default_idle+0x9/0x20 arch/x86/kernel/process.c:767
default_idle_call+0x72/0xb0 kernel/sched/idle.c:122
cpuidle_idle_call kernel/sched/idle.c:199 [inline]
do_idle+0x36a/0x5f0 kernel/sched/idle.c:352
cpu_startup_entry+0x43/0x60 kernel/sched/idle.c:451
start_secondary+0x101/0x110 arch/x86/kernel/smpboot.c:312
common_startup_64+0x13e/0x147
</TASK>
Reply: Based on the analysis of the stack traces and the kernel source code, here is the assessment of the bug:
**Root Cause Analysis:**
The bug report indicates an RCU stall where tasks are blocked on an RCU node, preventing the RCU grace period kthread from completing its work. The stack traces for the blocked tasks (`syz-executor`) show that they are in the process of freeing a large number of pages during file eviction/truncation (`shmem_undo_range` and `truncate_inode_pages_range`).
Specifically, the tasks are deep inside the `page_owner` tracking code (`__reset_page_owner`, `save_stack`, `unwind_next_frame`, and `__update_page_owner_free_handle`). `page_owner` is a kernel debugging feature that records the allocation and freeing stack traces for every single page in the system. Stack unwinding (`unwind_next_frame`) is a computationally expensive and slow operation.
When a process deletes or truncates a very large file, it frees thousands or millions of pages in a loop. With `page_owner` enabled, the kernel has to unwind and save the stack trace for every single page being freed. This massive overhead keeps the CPUs busy for an extended period (over 10.5 seconds in this report), which starves the `rcu_preempt` kthread and triggers the RCU stall detector.
**Security Impact:**
The `page_owner` feature is strictly a debugging and memory profiling tool (enabled via `CONFIG_PAGE_OWNER=y` and the `page_owner=on` boot parameter). It is **not enabled in production kernel builds** due to its severe performance overhead.
In a standard production environment without `page_owner` and other heavy debugging tools (like KASAN), freeing pages is an extremely fast operation. The loops in `shmem_undo_range` and `truncate_inode_pages_range` will complete quickly and will not cause an RCU stall or CPU lockup. There are no infinite loops in these functions, as the page cache iterators correctly advance their indices regardless of whether the pages are immediately freed.
Therefore, this is a false positive caused by the artificial overhead of debugging tools. It does not represent a Denial of Service (DoS) or an exploitable vulnerability in production systems.
**Trigger Vectors:**
While the bug itself is not present in production, the underlying code path (creating and deleting/truncating large files) is reachable by any unprivileged user, including those inside user namespaces or running within a VM guest.
|
| 6/2 |
2026/05/15 11:23 |
llm |
expert |
0m
Model:
Tokens:
input: 9703
output: 19
thoughts: 2492
Thoughts: **RCU Stall Investigation**
Okay, here's what I'm seeing. The bug report points to an RCU stall, specifically a "rcu_preempt detected stalls" issue. The system reports two tasks, P25104 and P28178, as blocked on an `rcu_node`. Let's grab those stack traces and see what they are doing.
Task 28178 appears to be deep inside page freeing. It goes through `shmem_evict_inode`, `shmem_truncate_range`, `folio_batch_release`, and ultimately calls `free_unref_folios` and `reset_page_owner`. Task 25104 has a similar path, starting with `ext4_evict_inode` which also leads to page freeing through `free_unref_folios` and `reset_page_owner`. Both are using `page_owner` which is a debugging tool and will definitely make the freeing take longer.
Now, the prompt says to ignore things like KASAN, KMSAN, and UBSAN. `page_owner` is a debugging feature, enabled by `CONFIG_PAGE_OWNER`. So, is the problem just slow freeing due to this debugging feature? Not necessarily. The `rcu_preempt` warning means these tasks were interrupted *within* an RCU read-side critical section, and aren't resuming.
But wait, is `free_unref_folios` actually holding an RCU read lock? Let me confirm. It's a key function in this stall, and it calls `folios_put_refs` and `__free_pages_prepare`. It seems possible an RCU read lock is being held somewhere in the process.
Okay, let's look for clues in the logs. Ah! The RCU preempt kthread itself seems to be starved. The log shows it's been starved for a long time. It says if the RCU kthread does not get CPU time OOM is expected. It's on CPU 1. Is that CPU actually idle? Yeah, the CPU is in a safe halt state. So, the kthread is supposed to be scheduled on CPU 1, but it's not getting any time. That's a red flag. The tasks are blocking the grace period.
Both are in `task_work_run` and moving through their file system's eviction paths, ultimately arriving at `free_unref_folios` with `page_owner` enabled. Is there an RCU read lock held in those paths? Let me dig into those functions: `shmem_undo_range` and `truncate_inode_pages_range`. The `find_lock_entries` function is called in `shmem_undo_range`, and that... uses an RCU read lock. The function calls `folio_batch_release` and then `folios_put_refs`.
But here's the thing: those tasks being preempted (marked as `b..l`) would only block if they had RCU read lock and were not resumed. `find_lock_entries` in `shmem_undo_range` may hold RCU lock, so this is where the problem lies. They are not actually holding RCU read locks at the same time, because if the locks were active, the RCU would be able to detect the blocked tasks. This must be the core issue.
|
| 7/2 |
2026/05/15 11:23 |
tool |
grepper |
0m
Args: map[Expression:folio_batch_release]
Results: map[Output:drivers/gpu/drm/drm_gem.c=633=static void drm_gem_check_release_batch(struct folio_batch *fbatch)
--
drivers/gpu/drm/drm_gem.c-635- check_move_unevictable_folios(fbatch);
drivers/gpu/drm/drm_gem.c:636: __folio_batch_release(fbatch);
drivers/gpu/drm/drm_gem.c-637- cond_resched();
--
drivers/gpu/drm/i915/gem/i915_gem_shmem.c=27=static void check_release_folio_batch(struct folio_batch *fbatch)
--
drivers/gpu/drm/i915/gem/i915_gem_shmem.c-29- check_move_unevictable_folios(fbatch);
drivers/gpu/drm/i915/gem/i915_gem_shmem.c:30: __folio_batch_release(fbatch);
drivers/gpu/drm/i915/gem/i915_gem_shmem.c-31- cond_resched();
--
drivers/gpu/drm/i915/i915_gpu_error.c=191=static void pool_fini(struct folio_batch *fbatch)
drivers/gpu/drm/i915/i915_gpu_error.c-192-{
drivers/gpu/drm/i915/i915_gpu_error.c:193: folio_batch_release(fbatch);
drivers/gpu/drm/i915/i915_gpu_error.c-194-}
--
fs/btrfs/compression.c=250=static noinline void end_compressed_writeback(const struct compressed_bio *cb)
--
fs/btrfs/compression.c-277- }
fs/btrfs/compression.c:278: folio_batch_release(&fbatch);
fs/btrfs/compression.c-279- }
--
fs/btrfs/extent_io.c=266=static void __process_folios_contig(struct address_space *mapping,
--
fs/btrfs/extent_io.c-287- }
fs/btrfs/extent_io.c:288: folio_batch_release(&fbatch);
fs/btrfs/extent_io.c-289- cond_resched();
--
fs/btrfs/extent_io.c=303=static noinline int lock_delalloc_folios(struct inode *inode,
--
fs/btrfs/extent_io.c-341- }
fs/btrfs/extent_io.c:342: folio_batch_release(&fbatch);
fs/btrfs/extent_io.c-343- cond_resched();
--
fs/btrfs/extent_io.c-347-out:
fs/btrfs/extent_io.c:348: folio_batch_release(&fbatch);
fs/btrfs/extent_io.c-349- if (processed_end > start)
--
fs/btrfs/extent_io.c=2427=static int extent_write_cache_pages(struct address_space *mapping,
--
fs/btrfs/extent_io.c-2565- }
fs/btrfs/extent_io.c:2566: folio_batch_release(&fbatch);
fs/btrfs/extent_io.c-2567- cond_resched();
--
fs/btrfs/file.c=2228=static bool check_range_has_page(struct inode *inode, u64 start, u64 end)
--
fs/btrfs/file.c-2267- }
fs/btrfs/file.c:2268: folio_batch_release(&fbatch);
fs/btrfs/file.c-2269- return ret;
--
fs/btrfs/tests/extent-io-tests.c=21=static noinline int process_page_range(struct inode *inode, u64 start, u64 end,
--
fs/btrfs/tests/extent-io-tests.c-47- }
fs/btrfs/tests/extent-io-tests.c:48: folio_batch_release(&fbatch);
fs/btrfs/tests/extent-io-tests.c-49- cond_resched();
--
fs/buffer.c=1635=void clean_bdev_aliases(struct block_device *bdev, sector_t block, sector_t len)
--
fs/buffer.c-1679- }
fs/buffer.c:1680: folio_batch_release(&fbatch);
fs/buffer.c-1681- cond_resched();
--
fs/ceph/addr.c=1034=void ceph_folio_batch_reinit(struct ceph_writeback_ctl *ceph_wbc)
fs/ceph/addr.c-1035-{
fs/ceph/addr.c:1036: folio_batch_release(&ceph_wbc->fbatch);
fs/ceph/addr.c-1037- ceph_folio_batch_init(ceph_wbc);
--
fs/ceph/addr.c=1606=void ceph_wait_until_current_writes_complete(struct address_space *mapping,
--
fs/ceph/addr.c-1630-
fs/ceph/addr.c:1631: folio_batch_release(&ceph_wbc->fbatch);
fs/ceph/addr.c-1632- cond_resched();
--
fs/ceph/addr.c=1640=static int ceph_writepages_start(struct address_space *mapping,
--
fs/ceph/addr.c-1741- ceph_wbc.fbatch.nr ? ceph_wbc.fbatch.folios[0] : NULL);
fs/ceph/addr.c:1742: folio_batch_release(&ceph_wbc.fbatch);
fs/ceph/addr.c-1743- }
--
fs/ext4/inode.c=1714=static void mpage_release_unused_pages(struct mpage_da_data *mpd,
--
fs/ext4/inode.c-1780- }
fs/ext4/inode.c:1781: folio_batch_release(&fbatch);
fs/ext4/inode.c-1782- }
--
fs/ext4/inode.c=2312=static int mpage_map_and_submit_buffers(struct mpage_da_data *mpd)
--
fs/ext4/inode.c-2350- }
fs/ext4/inode.c:2351: folio_batch_release(&fbatch);
fs/ext4/inode.c-2352- }
--
fs/ext4/inode.c-2357-out:
fs/ext4/inode.c:2358: folio_batch_release(&fbatch);
fs/ext4/inode.c-2359- return err;
--
fs/ext4/inode.c=2629=static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
--
fs/ext4/inode.c-2759- }
fs/ext4/inode.c:2760: folio_batch_release(&fbatch);
fs/ext4/inode.c-2761- cond_resched();
--
fs/ext4/inode.c-2767-out:
fs/ext4/inode.c:2768: folio_batch_release(&fbatch);
fs/ext4/inode.c-2769- if (handle)
--
fs/f2fs/checkpoint.c=608=long f2fs_sync_meta_pages(struct f2fs_sb_info *sbi, long nr_to_write,
--
fs/f2fs/checkpoint.c-633- folio_nr_pages(fbatch.folios[i-1])) {
fs/f2fs/checkpoint.c:634: folio_batch_release(&fbatch);
fs/f2fs/checkpoint.c-635- goto stop;
--
fs/f2fs/checkpoint.c-664- }
fs/f2fs/checkpoint.c:665: folio_batch_release(&fbatch);
fs/f2fs/checkpoint.c-666- cond_resched();
--
fs/f2fs/compress.c=1999=void f2fs_invalidate_compress_pages(struct f2fs_sb_info *sbi, nid_t ino)
--
fs/f2fs/compress.c-2034- }
fs/f2fs/compress.c:2035: folio_batch_release(&fbatch);
fs/f2fs/compress.c-2036- cond_resched();
--
fs/f2fs/data.c=3238=static int f2fs_write_cache_pages(struct address_space *mapping,
--
fs/f2fs/data.c-3331- index = folio->index + idx + 1;
fs/f2fs/data.c:3332: folio_batch_release(&fbatch);
fs/f2fs/data.c-3333- goto write;
--
fs/f2fs/data.c-3337- }
fs/f2fs/data.c:3338: folio_batch_release(&fbatch);
fs/f2fs/data.c-3339- goto again;
--
fs/f2fs/node.c=1676=static struct folio *last_fsync_dnode(struct f2fs_sb_info *sbi, nid_t ino)
--
fs/f2fs/node.c-1695- f2fs_folio_put(last_folio, false);
fs/f2fs/node.c:1696: folio_batch_release(&fbatch);
fs/f2fs/node.c-1697- return ERR_PTR(-EIO);
--
fs/f2fs/node.c-1726- }
fs/f2fs/node.c:1727: folio_batch_release(&fbatch);
fs/f2fs/node.c-1728- cond_resched();
--
fs/f2fs/node.c=1894=int f2fs_fsync_node_pages(struct f2fs_sb_info *sbi, struct inode *inode,
--
fs/f2fs/node.c-1927- f2fs_folio_put(last_folio, false);
fs/f2fs/node.c:1928: folio_batch_release(&fbatch);
fs/f2fs/node.c-1929- ret = -EIO;
--
fs/f2fs/node.c-1976- f2fs_folio_put(last_folio, false);
fs/f2fs/node.c:1977: folio_batch_release(&fbatch);
fs/f2fs/node.c-1978- ret = -EIO;
--
fs/f2fs/node.c-1985- f2fs_folio_put(folio, false);
fs/f2fs/node.c:1986: folio_batch_release(&fbatch);
fs/f2fs/node.c-1987- marked = true;
--
fs/f2fs/node.c-1990- }
fs/f2fs/node.c:1991: folio_batch_release(&fbatch);
fs/f2fs/node.c-1992- cond_resched();
--
fs/f2fs/node.c=2050=void f2fs_flush_inline_data(struct f2fs_sb_info *sbi)
--
fs/f2fs/node.c-2085- }
fs/f2fs/node.c:2086: folio_batch_release(&fbatch);
fs/f2fs/node.c-2087- cond_resched();
--
fs/f2fs/node.c=2091=int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
--
fs/f2fs/node.c-2176- wbc, do_balance, io_type, NULL)) {
fs/f2fs/node.c:2177: folio_batch_release(&fbatch);
fs/f2fs/node.c-2178- ret = -EIO;
--
fs/f2fs/node.c-2186- }
fs/f2fs/node.c:2187: folio_batch_release(&fbatch);
fs/f2fs/node.c-2188- cond_resched();
--
fs/gfs2/aops.c=283=static int gfs2_write_cache_jdata(struct address_space *mapping,
--
fs/gfs2/aops.c-331- ret = 0;
fs/gfs2/aops.c:332: folio_batch_release(&fbatch);
fs/gfs2/aops.c-333- cond_resched();
--
fs/hugetlbfs/inode.c=571=static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
--
fs/hugetlbfs/inode.c-601- }
fs/hugetlbfs/inode.c:602: folio_batch_release(&fbatch);
fs/hugetlbfs/inode.c-603- cond_resched();
--
fs/iomap/iter.c=9=static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
--
fs/iomap/iter.c-11- if (iter->iomap.flags & IOMAP_F_FOLIO_BATCH) {
fs/iomap/iter.c:12: folio_batch_release(iter->fbatch);
fs/iomap/iter.c-13- folio_batch_reinit(iter->fbatch);
--
fs/netfs/buffered_read.c=81=static ssize_t netfs_prepare_read_iterator(struct netfs_io_subrequest *subreq,
--
fs/netfs/buffered_read.c-108- }
fs/netfs/buffered_read.c:109: folio_batch_release(&put_batch);
fs/netfs/buffered_read.c-110- }
--
fs/netfs/misc.c=78=void netfs_free_folioq_buffer(struct folio_queue *fq)
--
fs/netfs/misc.c-94- if (folio_batch_add(&fbatch, folio))
fs/netfs/misc.c:95: folio_batch_release(&fbatch);
fs/netfs/misc.c-96- }
--
fs/netfs/misc.c-102-
fs/netfs/misc.c:103: folio_batch_release(&fbatch);
fs/netfs/misc.c-104-}
--
fs/netfs/rolling_buffer.c=116=ssize_t rolling_buffer_load_from_ra(struct rolling_buffer *roll,
--
fs/netfs/rolling_buffer.c-142- if (!folio_batch_add(put_batch, folio))
fs/netfs/rolling_buffer.c:143: folio_batch_release(put_batch);
fs/netfs/rolling_buffer.c-144- }
--
fs/netfs/rolling_buffer.c=197=void rolling_buffer_clear(struct rolling_buffer *roll)
--
fs/netfs/rolling_buffer.c-213- if (!folio_batch_add(&fbatch, folio))
fs/netfs/rolling_buffer.c:214: folio_batch_release(&fbatch);
fs/netfs/rolling_buffer.c-215- }
--
fs/netfs/rolling_buffer.c-220-
fs/netfs/rolling_buffer.c:221: folio_batch_release(&fbatch);
fs/netfs/rolling_buffer.c-222-}
--
fs/nilfs2/btree.c=2166=static void nilfs_btree_lookup_dirty_buffers(struct nilfs_bmap *btree,
--
fs/nilfs2/btree.c-2193- }
fs/nilfs2/btree.c:2194: folio_batch_release(&fbatch);
fs/nilfs2/btree.c-2195- cond_resched();
--
fs/nilfs2/page.c=243=int nilfs_copy_dirty_pages(struct address_space *dmap,
--
fs/nilfs2/page.c-281- }
fs/nilfs2/page.c:282: folio_batch_release(&fbatch);
fs/nilfs2/page.c-283- cond_resched();
--
fs/nilfs2/page.c=298=void nilfs_copy_back_pages(struct address_space *dmap,
--
fs/nilfs2/page.c-350- }
fs/nilfs2/page.c:351: folio_batch_release(&fbatch);
fs/nilfs2/page.c-352- cond_resched();
--
fs/nilfs2/page.c=361=void nilfs_clear_dirty_pages(struct address_space *mapping)
--
fs/nilfs2/page.c-385- }
fs/nilfs2/page.c:386: folio_batch_release(&fbatch);
fs/nilfs2/page.c-387- cond_resched();
--
fs/nilfs2/page.c=506=unsigned long nilfs_find_uncommitted_extent(struct inode *inode,
--
fs/nilfs2/page.c-558-
fs/nilfs2/page.c:559: folio_batch_release(&fbatch);
fs/nilfs2/page.c-560- cond_resched();
--
fs/nilfs2/page.c-564- folio_unlock(folio);
fs/nilfs2/page.c:565: folio_batch_release(&fbatch);
fs/nilfs2/page.c-566- return length;
--
fs/nilfs2/segment.c=698=static size_t nilfs_lookup_dirty_data_buffers(struct inode *inode,
--
fs/nilfs2/segment.c-748- folio_unlock(folio);
fs/nilfs2/segment.c:749: folio_batch_release(&fbatch);
fs/nilfs2/segment.c-750- cond_resched();
--
fs/nilfs2/segment.c-756- }
fs/nilfs2/segment.c:757: folio_batch_release(&fbatch);
fs/nilfs2/segment.c-758- cond_resched();
--
fs/nilfs2/segment.c=762=static void nilfs_lookup_dirty_node_buffers(struct inode *inode,
--
fs/nilfs2/segment.c-789- }
fs/nilfs2/segment.c:790: folio_batch_release(&fbatch);
fs/nilfs2/segment.c-791- cond_resched();
--
fs/ramfs/file-nommu.c=202=static unsigned long ramfs_nommu_get_unmapped_area(struct file *file,
--
fs/ramfs/file-nommu.c-249- if (nr_pages < lpages) {
fs/ramfs/file-nommu.c:250: folio_batch_release(&fbatch);
fs/ramfs/file-nommu.c-251- goto repeat; /* loop if pages are missing */
--
fs/ramfs/file-nommu.c-255-out_free:
fs/ramfs/file-nommu.c:256: folio_batch_release(&fbatch);
fs/ramfs/file-nommu.c-257-out:
--
include/linux/folio_batch.h=89=static inline struct folio *folio_batch_next(struct folio_batch *fbatch)
--
include/linux/folio_batch.h-95-
include/linux/folio_batch.h:96:void __folio_batch_release(struct folio_batch *fbatch);
include/linux/folio_batch.h-97-
include/linux/folio_batch.h:98:static inline void folio_batch_release(struct folio_batch *fbatch)
include/linux/folio_batch.h-99-{
include/linux/folio_batch.h-100- if (folio_batch_count(fbatch))
include/linux/folio_batch.h:101: __folio_batch_release(fbatch);
include/linux/folio_batch.h-102-}
--
include/linux/sunrpc/svc.h=509=static inline void svc_rqst_page_release(struct svc_rqst *rqstp,
--
include/linux/sunrpc/svc.h-512- if (!folio_batch_add(&rqstp->rq_fbatch, page_folio(page)))
include/linux/sunrpc/svc.h:513: __folio_batch_release(&rqstp->rq_fbatch);
include/linux/sunrpc/svc.h-514-}
--
mm/filemap.c=511=static void __filemap_fdatawait_range(struct address_space *mapping,
--
mm/filemap.c-534- }
mm/filemap.c:535: folio_batch_release(&fbatch);
mm/filemap.c-536- cond_resched();
--
mm/filemap.c=3054=ssize_t filemap_splice_read(struct file *in, loff_t *ppos,
--
mm/filemap.c-3135-
mm/filemap.c:3136: folio_batch_release(&fbatch);
mm/filemap.c-3137- } while (len);
--
mm/filemap.c-3139-out:
mm/filemap.c:3140: folio_batch_release(&fbatch);
mm/filemap.c-3141- file_accessed(in);
--
mm/gup.c=3436=long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
--
mm/gup.c-3495- if (try_grab_folio(folio, 1, FOLL_PIN)) {
mm/gup.c:3496: folio_batch_release(&fbatch);
mm/gup.c-3497- ret = -EINVAL;
--
mm/gup.c-3509- folio = NULL;
mm/gup.c:3510: folio_batch_release(&fbatch);
mm/gup.c-3511- if (!nr_found) {
--
mm/page-writeback.c=2418=static struct folio *writeback_get_folio(struct address_space *mapping,
--
mm/page-writeback.c-2425- if (!folio) {
mm/page-writeback.c:2426: folio_batch_release(&wbc->fbatch);
mm/page-writeback.c-2427- cond_resched();
--
mm/page-writeback.c=2471=struct folio *writeback_iter(struct address_space *mapping,
--
mm/page-writeback.c-2554- mapping->writeback_index = folio_next_index(folio);
mm/page-writeback.c:2555: folio_batch_release(&wbc->fbatch);
mm/page-writeback.c-2556- return NULL;
--
mm/shmem.c=1059=void shmem_unlock_mapping(struct address_space *mapping)
--
mm/shmem.c-1070- check_move_unevictable_folios(&fbatch);
mm/shmem.c:1071: folio_batch_release(&fbatch);
mm/shmem.c-1072- cond_resched();
--
mm/shmem.c=1108=static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
--
mm/shmem.c-1148- folio_batch_remove_exceptionals(&fbatch);
mm/shmem.c:1149: folio_batch_release(&fbatch);
mm/shmem.c-1150- cond_resched();
--
mm/shmem.c-1268- folio_batch_remove_exceptionals(&fbatch);
mm/shmem.c:1269: folio_batch_release(&fbatch);
mm/shmem.c-1270- }
--
mm/swap.c=1050=EXPORT_SYMBOL(release_pages);
--
mm/swap.c-1057- *
mm/swap.c:1058: * So __folio_batch_release() will drain those queues here.
mm/swap.c-1059- * folio_batch_move_lru() calls folios_put() directly to avoid
--
mm/swap.c-1061- */
mm/swap.c:1062:void __folio_batch_release(struct folio_batch *fbatch)
mm/swap.c-1063-{
--
mm/swap.c-1069-}
mm/swap.c:1070:EXPORT_SYMBOL(__folio_batch_release);
mm/swap.c-1071-
--
mm/truncate.c=366=void truncate_inode_pages_range(struct address_space *mapping,
--
mm/truncate.c-407- folio_unlock(fbatch.folios[i]);
mm/truncate.c:408: folio_batch_release(&fbatch);
mm/truncate.c-409- cond_resched();
--
mm/truncate.c-464- truncate_folio_batch_exceptionals(mapping, &fbatch, indices);
mm/truncate.c:465: folio_batch_release(&fbatch);
mm/truncate.c-466- }
--
mm/truncate.c=534=unsigned long mapping_try_invalidate(struct address_space *mapping,
--
mm/truncate.c-578- folio_batch_remove_exceptionals(&fbatch);
mm/truncate.c:579: folio_batch_release(&fbatch);
mm/truncate.c-580- cond_resched();
--
mm/truncate.c=676=int invalidate_inode_pages2_range(struct address_space *mapping,
--
mm/truncate.c-735- folio_batch_remove_exceptionals(&fbatch);
mm/truncate.c:736: folio_batch_release(&fbatch);
mm/truncate.c-737- cond_resched();
--
net/sunrpc/svc.c=687=svc_rqst_free(struct svc_rqst *rqstp)
net/sunrpc/svc.c-688-{
net/sunrpc/svc.c:689: folio_batch_release(&rqstp->rq_fbatch);
net/sunrpc/svc.c-690- kfree(rqstp->rq_bvec);
--
net/sunrpc/svc.c=996=void svc_rqst_release_pages(struct svc_rqst *rqstp)
--
net/sunrpc/svc.c-1003- page_folio(*pp)))
net/sunrpc/svc.c:1004: __folio_batch_release(&rqstp->rq_fbatch);
net/sunrpc/svc.c-1005- *pp = NULL;
--
net/sunrpc/svc.c-1008- if (rqstp->rq_fbatch.nr)
net/sunrpc/svc.c:1009: __folio_batch_release(&rqstp->rq_fbatch);
net/sunrpc/svc.c-1010-}
]
|
| 8/2 |
2026/05/15 11:23 |
llm |
expert |
0m
Model:
Tokens:
input: 20354
output: 40
thoughts: 363
Thoughts: **RCU Stalls and Preemption: Unraveling the Mystery**
Okay, so I'm looking at this code and trying to understand why we might see RCU stalls. It seems like `folio_batch_release` is the culprit, and it's called through `__folio_batch_release` and `folios_put_refs`. Now, here's the tricky part: within `shmem_undo_range`, `folio_batch_release` is called, and then `cond_resched()` immediately follows. My first thought was, "If `folio_batch_release` takes a significant amount of time, could preemption cause it to block RCU grace periods?" But wait, that doesn't quite make sense. If it's preempted, it's *not* holding the RCU read lock, right? So, why would it be blocking the RCU grace period?
Ah, then it hits me: the stall message shows tasks blocked on an RCU node. This screams "preemption in an RCU read-side critical section!" So, the question is, *where* is the RCU read lock being held? I need to track down the source. Let's trace it back to `shmem_undo_range` and `truncate_inode_pages_range`. Looking at `shmem_undo_range`, specifically, I see the `folio_batch_init` and then a loop. Inside this loop we have `find_lock_entries`. That's where I need to focus my attention. It looks like `find_lock_entries` is the key to unraveling this. I need to understand what's happening within that function, to understand how we are possibly entering into an RCU read-side critical section.
|
| 9/2 |
2026/05/15 11:24 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/shmem.c IncludeLines:true Name:find_lock_entries]
Results: map[SourceCode:2145: /**
2146: * find_lock_entries - Find a batch of pagecache entries.
2147: * @mapping: The address_space to search.
2148: * @start: The starting page cache index.
2149: * @end: The final page index (inclusive).
2150: * @fbatch: Where the resulting entries are placed.
2151: * @indices: The cache indices of the entries in @fbatch.
2152: *
2153: * find_lock_entries() will return a batch of entries from @mapping.
2154: * Swap, shadow and DAX entries are included. Folios are returned
2155: * locked and with an incremented refcount. Folios which are locked
2156: * by somebody else or under writeback are skipped. Folios which are
2157: * partially outside the range are not returned.
2158: *
2159: * The entries have ascending indexes. The indices may not be consecutive
2160: * due to not-present entries, large folios, folios which could not be
2161: * locked or folios under writeback.
2162: *
2163: * Return: The number of entries which were found.
2164: */
2165: unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start,
2166: pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices)
2167: {
2168: XA_STATE(xas, &mapping->i_pages, *start);
2169: struct folio *folio;
2170:
2171: rcu_read_lock();
2172: while ((folio = find_get_entry(&xas, end, XA_PRESENT))) {
2173: unsigned long base;
2174: unsigned long nr;
2175:
2176: if (!xa_is_value(folio)) {
2177: nr = folio_nr_pages(folio);
2178: base = folio->index;
2179: /* Omit large folio which begins before the start */
2180: if (base < *start)
2181: goto put;
2182: /* Omit large folio which extends beyond the end */
2183: if (base + nr - 1 > end)
2184: goto put;
2185: if (!folio_trylock(folio))
2186: goto put;
2187: if (folio->mapping != mapping ||
2188: folio_test_writeback(folio))
2189: goto unlock;
2190: VM_BUG_ON_FOLIO(!folio_contains(folio, xas.xa_index),
2191: folio);
2192: } else {
2193: nr = 1 << xas_get_order(&xas);
2194: base = xas.xa_index & ~(nr - 1);
2195: /* Omit order>0 value which begins before the start */
2196: if (base < *start)
2197: continue;
2198: /* Omit order>0 value which extends beyond the end */
2199: if (base + nr - 1 > end)
2200: break;
2201: }
2202:
2203: /* Update start now so that last update is correct on return */
2204: *start = base + nr;
2205: indices[fbatch->nr] = xas.xa_index;
2206: if (!folio_batch_add(fbatch, folio))
2207: break;
2208: continue;
2209: unlock:
2210: folio_unlock(folio);
2211: put:
2212: folio_put(folio);
2213: }
2214: rcu_read_unlock();
2215:
2216: return folio_batch_count(fbatch);
2217: }
SourceFile:mm/filemap.c]
|
| 10/2 |
2026/05/15 11:24 |
llm |
expert |
0m
Model:
Tokens:
input: 22279
output: 41
thoughts: 355
Thoughts: **RCU Stall in Shmem: A Deep Dive**
Okay, so I'm looking at this code and immediately see `find_lock_entries` using `rcu_read_lock()` and `rcu_read_unlock()`. Standard RCU practice. But then, hold on... inside `find_lock_entries`, I see `folio_put(folio)` being called while *still* holding that RCU lock. Now, I know `folio_put` decrements the folio's reference count. If that drops it to zero, it's going to free the folio, right?
But, wait... `find_get_entry` increments the refcount *before* that call. So, in the normal course of events, `folio_put` should just decrement the count that was incremented. It *shouldn't* free the folio, unless there's concurrent dropping of the last reference.
Okay, so the scenario where the folio *is* freed, is if someone else, concurrently, dropped the last reference. Then `folio_put` frees it. And freeing it calls `free_unref_folios`, which in turn, calls `reset_page_owner`. And I know that `reset_page_owner` does stack unwinding. Stack unwinding is slow. Very slow.
And here's the kicker: If it gets preempted *during* that slow operation, it's preempted *while still holding* that `rcu_read_lock()`. Bingo! That's going to cause an RCU stall. We've got a potential problem here.
Okay, let's look at the stack trace. Aha! It's showing `shmem_undo_range+0x52c/0x1660 mm/shmem.c:1149`. Line 1149? Let's go there...
```c
folio_batch_release(&fbatch);
```
Whoa, hold on. `folio_batch_release` is being called *outside* the `find_lock_entries` function. Now, I need to understand `shmem_undo_range` better... Let's check that out.
|
| 11/2 |
2026/05/15 11:24 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/shmem.c IncludeLines:true Name:shmem_undo_range]
Results: map[SourceCode:1104: /*
1105: * Remove range of pages and swap entries from page cache, and free them.
1106: * If !unfalloc, truncate or punch hole; if unfalloc, undo failed fallocate.
1107: */
1108: static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
1109: bool unfalloc)
1110: {
1111: struct address_space *mapping = inode->i_mapping;
1112: struct shmem_inode_info *info = SHMEM_I(inode);
1113: pgoff_t start = (lstart + PAGE_SIZE - 1) >> PAGE_SHIFT;
1114: pgoff_t end = (lend + 1) >> PAGE_SHIFT;
1115: struct folio_batch fbatch;
1116: pgoff_t indices[FOLIO_BATCH_SIZE];
1117: struct folio *folio;
1118: bool same_folio;
1119: long nr_swaps_freed = 0;
1120: pgoff_t index;
1121: int i;
1122:
1123: if (lend == -1)
1124: end = -1; /* unsigned, so actually very big */
1125:
1126: if (info->fallocend > start && info->fallocend <= end && !unfalloc)
1127: info->fallocend = start;
1128:
1129: folio_batch_init(&fbatch);
1130: index = start;
1131: while (index < end && find_lock_entries(mapping, &index, end - 1,
1132: &fbatch, indices)) {
1133: for (i = 0; i < folio_batch_count(&fbatch); i++) {
1134: folio = fbatch.folios[i];
1135:
1136: if (xa_is_value(folio)) {
1137: if (unfalloc)
1138: continue;
1139: nr_swaps_freed += shmem_free_swap(mapping, indices[i],
1140: end - 1, folio);
1141: continue;
1142: }
1143:
1144: if (!unfalloc || !folio_test_uptodate(folio))
1145: truncate_inode_folio(mapping, folio);
1146: folio_unlock(folio);
1147: }
1148: folio_batch_remove_exceptionals(&fbatch);
1149: folio_batch_release(&fbatch);
1150: cond_resched();
1151: }
1152:
1153: /*
1154: * When undoing a failed fallocate, we want none of the partial folio
1155: * zeroing and splitting below, but shall want to truncate the whole
1156: * folio when !uptodate indicates that it was added by this fallocate,
1157: * even when [lstart, lend] covers only a part of the folio.
1158: */
1159: if (unfalloc)
1160: goto whole_folios;
1161:
1162: same_folio = (lstart >> PAGE_SHIFT) == (lend >> PAGE_SHIFT);
1163: folio = shmem_get_partial_folio(inode, lstart >> PAGE_SHIFT);
1164: if (folio) {
1165: same_folio = lend < folio_next_pos(folio);
1166: folio_mark_dirty(folio);
1167: if (!truncate_inode_partial_folio(folio, lstart, lend)) {
1168: start = folio_next_index(folio);
1169: if (same_folio)
1170: end = folio->index;
1171: }
1172: folio_unlock(folio);
1173: folio_put(folio);
1174: folio = NULL;
1175: }
1176:
1177: if (!same_folio)
1178: folio = shmem_get_partial_folio(inode, lend >> PAGE_SHIFT);
1179: if (folio) {
1180: folio_mark_dirty(folio);
1181: if (!truncate_inode_partial_folio(folio, lstart, lend))
1182: end = folio->index;
1183: folio_unlock(folio);
1184: folio_put(folio);
1185: }
1186:
1187: whole_folios:
1188:
1189: index = start;
1190: while (index < end) {
1191: cond_resched();
1192:
1193: if (!find_get_entries(mapping, &index, end - 1, &fbatch,
1194: indices)) {
1195: /* If all gone or hole-punch or unfalloc, we're done */
1196: if (index == start || end != -1)
1197: break;
1198: /* But if truncating, restart to make sure all gone */
1199: index = start;
1200: continue;
1201: }
1202: for (i = 0; i < folio_batch_count(&fbatch); i++) {
1203: folio = fbatch.folios[i];
1204:
1205: if (xa_is_value(folio)) {
1206: int order;
1207: long swaps_freed;
1208:
1209: if (unfalloc)
1210: continue;
1211: swaps_freed = shmem_free_swap(mapping, indices[i],
1212: end - 1, folio);
1213: if (!swaps_freed) {
1214: pgoff_t base = indices[i];
1215:
1216: order = shmem_confirm_swap(mapping, indices[i],
1217: radix_to_swp_entry(folio));
1218: /*
1219: * If found a large swap entry cross the end or start
1220: * border, skip it as the truncate_inode_partial_folio
1221: * above should have at least zerod its content once.
1222: */
1223: if (order > 0) {
1224: base = round_down(base, 1 << order);
1225: if (base < start || base + (1 << order) > end)
1226: continue;
1227: }
1228: /* Swap was replaced by page or extended, retry */
1229: index = base;
1230: break;
1231: }
1232: nr_swaps_freed += swaps_freed;
1233: continue;
1234: }
1235:
1236: folio_lock(folio);
1237:
1238: if (!unfalloc || !folio_test_uptodate(folio)) {
1239: if (folio_mapping(folio) != mapping) {
1240: /* Page was replaced by swap: retry */
1241: folio_unlock(folio);
1242: index = indices[i];
1243: break;
1244: }
1245: VM_BUG_ON_FOLIO(folio_test_writeback(folio),
1246: folio);
1247:
1248: if (!folio_test_large(folio)) {
1249: truncate_inode_folio(mapping, folio);
1250: } else if (truncate_inode_partial_folio(folio, lstart, lend)) {
1251: /*
1252: * If we split a page, reset the loop so
1253: * that we pick up the new sub pages.
1254: * Otherwise the THP was entirely
1255: * dropped or the target range was
1256: * zeroed, so just continue the loop as
1257: * is.
1258: */
1259: if (!folio_test_large(folio)) {
1260: folio_unlock(folio);
1261: index = start;
1262: break;
1263: }
1264: }
1265: }
1266: folio_unlock(folio);
1267: }
1268: folio_batch_remove_exceptionals(&fbatch);
1269: folio_batch_release(&fbatch);
1270: }
1271:
1272: shmem_recalc_inode(inode, 0, -nr_swaps_freed);
1273: }
SourceFile:mm/shmem.c]
|
| 12/2 |
2026/05/15 11:24 |
llm |
expert |
0m
Model:
Tokens:
input: 25670
output: 40
thoughts: 183
Thoughts: **Investigating the RCU Grace Period Blocking in `folio_batch_release`**
Okay, hold on a second. It just hit me that `folio_batch_release` is actually called outside of `find_lock_entries`. And `find_lock_entries` explicitly releases the `rcu_read_lock()` before returning. That means `folio_batch_release` is *not* guaranteed to be called with `rcu_read_lock()` held, which raises a major question mark. If that lock isn't held, then why on earth is `folio_batch_release` blocking the RCU grace period? Something isn't adding up here, and I need to track down the source of this behavior.
My immediate next steps are to dive into the call chain. I need to understand if there is a problem with the lock. I'm going to carefully scrutinize `folios_put_refs`, `free_unref_folios`, and `reset_page_owner`. I'm especially interested in confirming that `rcu_read_lock` is *not* held anywhere within `folio_batch_release` and its children.
Let's start from the top. I need to trace the call flow from `folio_batch_release` down to `__folio_batch_release`, then down to `folios_put_refs` and finally, down to `free_unref_folios`. This should reveal where the RCU interaction is happening. Time to dig in and unravel this puzzle.
|
| 13/2 |
2026/05/15 11:25 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/swap.c IncludeLines:true Name:__folio_batch_release]
Results: map[SourceCode:1052: /*
1053: * The folios which we're about to release may be in the deferred lru-addition
1054: * queues. That would prevent them from really being freed right now. That's
1055: * OK from a correctness point of view but is inefficient - those folios may be
1056: * cache-warm and we want to give them back to the page allocator ASAP.
1057: *
1058: * So __folio_batch_release() will drain those queues here.
1059: * folio_batch_move_lru() calls folios_put() directly to avoid
1060: * mutual recursion.
1061: */
1062: void __folio_batch_release(struct folio_batch *fbatch)
1063: {
1064: if (!fbatch->percpu_pvec_drained) {
1065: lru_add_drain();
1066: fbatch->percpu_pvec_drained = true;
1067: }
1068: folios_put(fbatch);
1069: }
SourceFile:mm/swap.c]
|
| 14/2 |
2026/05/15 11:25 |
llm |
expert |
0m
Model:
Tokens:
input: 26546
output: 39
thoughts: 13
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience, and written as if they were my own immediate processing:
**Direct Method Invocation Analysis**
Right, so `folios_put`...that's the external facing entry point for updating or creating folio data. Now, a crucial detail emerges: it directly *calls* `folios_put_refs`. This is a pretty important architectural choice, and immediately flags some design considerations. It strongly suggests that `folios_put_refs` is likely responsible for the core, low-level folio data management. `folios_put` itself is probably handling things like input validation, possibly some higher-level object setup, and maybe access control checks before deferring the actual data operations to the lower-level function. The implication is also that `folios_put_refs` would likely provide more granular control and a common interface to access folios and possibly their constituent parts. I need to look at the relationship between these two methods to understand the complete execution path. This kind of arrangement often hints at the use of a more complex composite pattern where various folio objects can be combined and shared. My next step will be to explore `folios_put_refs` to understand its role and impact.
|
| 15/2 |
2026/05/15 11:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/swap.c IncludeLines:true Name:folios_put_refs]
Results: map[SourceCode: 942: /**
943: * folios_put_refs - Reduce the reference count on a batch of folios.
944: * @folios: The folios.
945: * @refs: The number of refs to subtract from each folio.
946: *
947: * Like folio_put(), but for a batch of folios. This is more efficient
948: * than writing the loop yourself as it will optimise the locks which need
949: * to be taken if the folios are freed. The folios batch is returned
950: * empty and ready to be reused for another batch; there is no need
951: * to reinitialise it. If @refs is NULL, we subtract one from each
952: * folio refcount.
953: *
954: * Context: May be called in process or interrupt context, but not in NMI
955: * context. May be called while holding a spinlock.
956: */
957: void folios_put_refs(struct folio_batch *folios, unsigned int *refs)
958: {
959: int i, j;
960: struct lruvec *lruvec = NULL;
961: unsigned long flags = 0;
962:
963: for (i = 0, j = 0; i < folios->nr; i++) {
964: struct folio *folio = folios->folios[i];
965: unsigned int nr_refs = refs ? refs[i] : 1;
966:
967: if (is_huge_zero_folio(folio))
968: continue;
969:
970: if (folio_is_zone_device(folio)) {
971: if (lruvec) {
972: lruvec_unlock_irqrestore(lruvec, flags);
973: lruvec = NULL;
974: }
975: if (folio_ref_sub_and_test(folio, nr_refs))
976: free_zone_device_folio(folio);
977: continue;
978: }
979:
980: if (!folio_ref_sub_and_test(folio, nr_refs))
981: continue;
982:
983: /* hugetlb has its own memcg */
984: if (folio_test_hugetlb(folio)) {
985: if (lruvec) {
986: lruvec_unlock_irqrestore(lruvec, flags);
987: lruvec = NULL;
988: }
989: free_huge_folio(folio);
990: continue;
991: }
992: folio_unqueue_deferred_split(folio);
993: __page_cache_release(folio, &lruvec, &flags);
994:
995: if (j != i)
996: folios->folios[j] = folio;
997: j++;
998: }
999: if (lruvec)
1000: lruvec_unlock_irqrestore(lruvec, flags);
1001: if (!j) {
1002: folio_batch_reinit(folios);
1003: return;
1004: }
1005:
1006: folios->nr = j;
1007: mem_cgroup_uncharge_folios(folios);
1008: free_unref_folios(folios);
1009: }
SourceFile:mm/swap.c]
|
| 16/2 |
2026/05/15 11:26 |
llm |
expert |
0m
Model:
Tokens:
input: 27904
output: 42
thoughts: 37
Thoughts: Okay, here's my take on those thoughts, framed as my own:
**Investigating a Folio Lifecycle Concern**
Alright, let's see, `folios_put_refs` triggers `free_unref_folios`. My immediate thought is, "Okay, that's the expected clean-up path for folios that are no longer referenced." But that begs a critical question, and one I need to be absolutely certain about. Does `free_unref_folios` operate under the protection of an `rcu_read_lock`? If not, there's a serious potential for a race condition when accessing folio metadata, and this whole thing could be a ticking time bomb! I need to dig into the code and verify that the necessary synchronization is in place, because a memory management bug here would be catastrophic.
|
| 17/2 |
2026/05/15 11:26 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/page_alloc.c IncludeLines:true Name:free_unref_folios]
Results: map[SourceCode:2989: /*
2990: * Free a batch of folios
2991: */
2992: void free_unref_folios(struct folio_batch *folios)
2993: {
2994: struct per_cpu_pages *pcp = NULL;
2995: struct zone *locked_zone = NULL;
2996: int i, j;
2997:
2998: /* Prepare folios for freeing */
2999: for (i = 0, j = 0; i < folios->nr; i++) {
3000: struct folio *folio = folios->folios[i];
3001: unsigned long pfn = folio_pfn(folio);
3002: unsigned int order = folio_order(folio);
3003:
3004: if (!__free_pages_prepare(&folio->page, order, FPI_NONE))
3005: continue;
3006: /*
3007: * Free orders not handled on the PCP directly to the
3008: * allocator.
3009: */
3010: if (!pcp_allowed_order(order)) {
3011: free_one_page(folio_zone(folio), &folio->page,
3012: pfn, order, FPI_NONE);
3013: continue;
3014: }
3015: folio->private = (void *)(unsigned long)order;
3016: if (j != i)
3017: folios->folios[j] = folio;
3018: j++;
3019: }
3020: folios->nr = j;
3021:
3022: for (i = 0; i < folios->nr; i++) {
3023: struct folio *folio = folios->folios[i];
3024: struct zone *zone = folio_zone(folio);
3025: unsigned long pfn = folio_pfn(folio);
3026: unsigned int order = (unsigned long)folio->private;
3027: int migratetype;
3028:
3029: folio->private = NULL;
3030: migratetype = get_pfnblock_migratetype(&folio->page, pfn);
3031:
3032: /* Different zone requires a different pcp lock */
3033: if (zone != locked_zone ||
3034: is_migrate_isolate(migratetype)) {
3035: if (pcp) {
3036: pcp_spin_unlock(pcp);
3037: locked_zone = NULL;
3038: pcp = NULL;
3039: }
3040:
3041: /*
3042: * Free isolated pages directly to the
3043: * allocator, see comment in free_frozen_pages.
3044: */
3045: if (is_migrate_isolate(migratetype)) {
3046: free_one_page(zone, &folio->page, pfn,
3047: order, FPI_NONE);
3048: continue;
3049: }
3050:
3051: /*
3052: * trylock is necessary as folios may be getting freed
3053: * from IRQ or SoftIRQ context after an IO completion.
3054: */
3055: pcp = pcp_spin_trylock(zone->per_cpu_pageset);
3056: if (unlikely(!pcp)) {
3057: free_one_page(zone, &folio->page, pfn,
3058: order, FPI_NONE);
3059: continue;
3060: }
3061: locked_zone = zone;
3062: }
3063:
3064: /*
3065: * Non-isolated types over MIGRATE_PCPTYPES get added
3066: * to the MIGRATE_MOVABLE pcp list.
3067: */
3068: if (unlikely(migratetype >= MIGRATE_PCPTYPES))
3069: migratetype = MIGRATE_MOVABLE;
3070:
3071: trace_mm_page_free_batched(&folio->page);
3072: if (!free_frozen_page_commit(zone, pcp, &folio->page,
3073: migratetype, order, FPI_NONE)) {
3074: pcp = NULL;
3075: locked_zone = NULL;
3076: }
3077: }
3078:
3079: if (pcp)
3080: pcp_spin_unlock(pcp);
3081: folio_batch_reinit(folios);
3082: }
SourceFile:mm/page_alloc.c]
|
| 18/2 |
2026/05/15 11:26 |
llm |
expert |
0m
Model:
Tokens:
input: 29523
output: 40
thoughts: 2011
Thoughts: **RCU Stall Investigation: A Deep Dive**
Okay, so I'm looking at this RCU stall report and need to figure out what's going on. The core issue seems to be a task being blocked on the RCU node, specifically related to `free_unref_folios` and indirectly, `reset_page_owner`. The initial thought is to check if an `rcu_read_lock` is held, and there isn't one directly around these functions. But, the stall detector is screaming about blocked tasks in level-0 `rcu_node`. Why are these tasks being put on the `blkd_tasks` list, where they are blocked while in an RCU read-side critical section? Where is the `rcu_read_lock` hiding?
Tracing the call stack back from `task_work_run` and `syscall_exit_to_user_mode` doesn't reveal any obvious lock holders that should be causing this. Then, I considered that a task work may have called and forgot to unlock, but that does not seem to be the case. I then had a hunch. Could `rcu_read_lock` be held inside `__fput` or called by a function in `__fput`'s call chain? And the answer is `find_lock_entries`.
Here's the critical bit: `find_lock_entries` holds `rcu_read_lock`, and it can call `folio_put` if a check fails. `folio_put` is responsible for decrementing the folio's reference count. Normally, this isn't an issue. However, if the folio is *concurrently* removed from the page cache, then the refcount *might* drop to zero within the `rcu_read_lock` critical section, leading to the folio's immediate deallocation. Deallocation then kicks off `free_unref_folios` which calls `reset_page_owner`, which uses stack unwinding.
This is the key. The *slow* `reset_page_owner` operation, with its stack unwinding, is the bottleneck, and being preempted during that while holding the `rcu_read_lock` is the cause of the RCU stall. If the system has debugging options like `page_owner` enabled, it makes this operation *very* slow, which explains the observed 10.5-second stall reported by syzbot. In a production environment, with `page_owner` disabled, this deallocation would be nearly instantaneous and undetectable.
Okay, let's look at the other stack trace. The second reported task, P28178, is at `shmem_undo_range+0x52c/0x1660`, which is line 1149: `folio_batch_release(&fbatch);`. But this *should* be outside `find_lock_entries`, meaning no RCU lock is held! So why is it blocked on the RCU node? This makes no sense. The RCU stall detector is telling us the task was preempted within an RCU read-side critical section, but it appears not to be in one.
Could the caller of `shmem_undo_range` be holding the lock? Trace the call stack: `shmem_truncate_range` -> `shmem_evict_inode` -> `evict`. None of these hold the lock, so what's going on? Perhaps the stack trace was taken *after* it resumed from being preempted and not during the RCU critical section? But no, the task state is `R`, meaning it's running. Why does RCU say it's blocked?
The RCU stall message means the task *was* preempted during the RCU read side, and it's *still* on the `blkd_tasks` list, even though it's now in state `R` - that the `blkd_tasks` list is stale. This means they are still in the critical section, *or* the stall was printed before the task was fully out of the section. If it's at `folio_batch_release`, it's *not* in `find_lock_entries`. Is there a lock leak? Is `rcu_read_lock` possibly being leaked in `find_lock_entries`?
|
| 19/2 |
2026/05/15 11:27 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/filemap.c IncludeLines:true Name:find_lock_entries]
Results: map[SourceCode:2145: /**
2146: * find_lock_entries - Find a batch of pagecache entries.
2147: * @mapping: The address_space to search.
2148: * @start: The starting page cache index.
2149: * @end: The final page index (inclusive).
2150: * @fbatch: Where the resulting entries are placed.
2151: * @indices: The cache indices of the entries in @fbatch.
2152: *
2153: * find_lock_entries() will return a batch of entries from @mapping.
2154: * Swap, shadow and DAX entries are included. Folios are returned
2155: * locked and with an incremented refcount. Folios which are locked
2156: * by somebody else or under writeback are skipped. Folios which are
2157: * partially outside the range are not returned.
2158: *
2159: * The entries have ascending indexes. The indices may not be consecutive
2160: * due to not-present entries, large folios, folios which could not be
2161: * locked or folios under writeback.
2162: *
2163: * Return: The number of entries which were found.
2164: */
2165: unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start,
2166: pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices)
2167: {
2168: XA_STATE(xas, &mapping->i_pages, *start);
2169: struct folio *folio;
2170:
2171: rcu_read_lock();
2172: while ((folio = find_get_entry(&xas, end, XA_PRESENT))) {
2173: unsigned long base;
2174: unsigned long nr;
2175:
2176: if (!xa_is_value(folio)) {
2177: nr = folio_nr_pages(folio);
2178: base = folio->index;
2179: /* Omit large folio which begins before the start */
2180: if (base < *start)
2181: goto put;
2182: /* Omit large folio which extends beyond the end */
2183: if (base + nr - 1 > end)
2184: goto put;
2185: if (!folio_trylock(folio))
2186: goto put;
2187: if (folio->mapping != mapping ||
2188: folio_test_writeback(folio))
2189: goto unlock;
2190: VM_BUG_ON_FOLIO(!folio_contains(folio, xas.xa_index),
2191: folio);
2192: } else {
2193: nr = 1 << xas_get_order(&xas);
2194: base = xas.xa_index & ~(nr - 1);
2195: /* Omit order>0 value which begins before the start */
2196: if (base < *start)
2197: continue;
2198: /* Omit order>0 value which extends beyond the end */
2199: if (base + nr - 1 > end)
2200: break;
2201: }
2202:
2203: /* Update start now so that last update is correct on return */
2204: *start = base + nr;
2205: indices[fbatch->nr] = xas.xa_index;
2206: if (!folio_batch_add(fbatch, folio))
2207: break;
2208: continue;
2209: unlock:
2210: folio_unlock(folio);
2211: put:
2212: folio_put(folio);
2213: }
2214: rcu_read_unlock();
2215:
2216: return folio_batch_count(fbatch);
2217: }
SourceFile:mm/filemap.c]
|
| 20/2 |
2026/05/15 11:27 |
llm |
expert |
0m
Model:
Tokens:
input: 33653
output: 41
thoughts: 1333
Thoughts: **RCU Stall Detector Mystery**
Okay, so I'm looking at this RCU stall detector message, and it's got me puzzled. First thought: `find_lock_entries` isn't leaking an RCU read-side critical section, right? But then I consider the possibility of `folio_put` being called inside `find_lock_entries` which could free a folio. If that happens, it calls `free_unref_folios`, which leads to `__free_pages_prepare`, and that calls `reset_page_owner`, which causes stack unwinding. And this stack unwinding can take a *while*.
A long stack unwind could be preempted. If that happens, the task would get added to the RCU node's `blkd_tasks` list. Then it would resume, finish unwinding, return to `find_lock_entries`, then call `rcu_read_unlock()` and exit `find_lock_entries`, move to `shmem_undo_range`, and *then* call `folio_batch_release`, which frees even more folios. More folio freeing also takes time (due to `page_owner`), but... if the RCU stall detector fires while the task is *in* `folio_batch_release`, that's where the problem arises. It prints the task's stack trace.
But here's the kicker: I thought `rcu_read_unlock()` removes the task from `blkd_tasks`. If it's *still* on the list, it *must* be inside the RCU read-side critical section, right? But the stack trace shows it's in `folio_batch_release`, which is *outside*! How is this even possible? Is it a delay in `rcu_read_unlock()`? But no, it's synchronous.
Maybe the task was preempted *during* `find_lock_entries`, was added to the `blkd_tasks` list, *then* resumed, called `rcu_read_unlock()`, and the RCU stall detector printed the message afterward? The stall detector runs on a timer interrupt. It detects a stall, prints the message about stalled tasks, and then sends NMIs to all CPUs. I check: Tasks 25104 and 28178 aren't *currently* running on any CPU! CPU 1 is idle, and CPU 0 is running the stall detector. So, they are runnable (`R` state), but not executing.
If they are runnable but not executing, the stack traces come from their *saved context*. Their saved context is the point where they were preempted! Let's examine those. Task 28178 was preempted by the timer interrupt: it was in `unwind_next_frame`. Task 25104 was preempted by the timer interrupt too, and was in `__nr_to_section`. Both were preempted by a timer interrupt (`asm_sysvec_apic_timer_interrupt`)! And crucially, they were preempted in `folio_batch_release` -> `folios_put_refs` -> `free_unref_folios` -> `reset_page_owner`.
But, if they were preempted inside `folio_batch_release`, that means they were NOT holding `rcu_read_lock()`! So why are they on the `blkd_tasks` list?!
Could `rcu_read_lock()` actually *still* be held? Let me re-examine `shmem_undo_range`... No. Is there any other `rcu_read_lock()`? No. What about `truncate_inode_pages_range` (for task 25104)? This is odd. Very odd indeed.
|
| 21/2 |
2026/05/15 11:27 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/truncate.c IncludeLines:true Name:truncate_inode_pages_range]
Results: map[SourceCode: 342: /**
343: * truncate_inode_pages_range - truncate range of pages specified by start & end byte offsets
344: * @mapping: mapping to truncate
345: * @lstart: offset from which to truncate
346: * @lend: offset to which to truncate (inclusive)
347: *
348: * Truncate the page cache, removing the pages that are between
349: * specified offsets (and zeroing out partial pages
350: * if lstart or lend + 1 is not page aligned).
351: *
352: * Truncate takes two passes - the first pass is nonblocking. It will not
353: * block on page locks and it will not block on writeback. The second pass
354: * will wait. This is to prevent as much IO as possible in the affected region.
355: * The first pass will remove most pages, so the search cost of the second pass
356: * is low.
357: *
358: * We pass down the cache-hot hint to the page freeing code. Even if the
359: * mapping is large, it is probably the case that the final pages are the most
360: * recently touched, and freeing happens in ascending file offset order.
361: *
362: * Note that since ->invalidate_folio() accepts range to invalidate
363: * truncate_inode_pages_range is able to handle cases where lend + 1 is not
364: * page aligned properly.
365: */
366: void truncate_inode_pages_range(struct address_space *mapping,
367: loff_t lstart, uoff_t lend)
368: {
369: pgoff_t start; /* inclusive */
370: pgoff_t end; /* exclusive */
371: struct folio_batch fbatch;
372: pgoff_t indices[FOLIO_BATCH_SIZE];
373: pgoff_t index;
374: int i;
375: struct folio *folio;
376: bool same_folio;
377:
378: if (mapping_empty(mapping))
379: return;
380:
381: /*
382: * 'start' and 'end' always covers the range of pages to be fully
383: * truncated. Partial pages are covered with 'partial_start' at the
384: * start of the range and 'partial_end' at the end of the range.
385: * Note that 'end' is exclusive while 'lend' is inclusive.
386: */
387: start = (lstart + PAGE_SIZE - 1) >> PAGE_SHIFT;
388: if (lend == -1)
389: /*
390: * lend == -1 indicates end-of-file so we have to set 'end'
391: * to the highest possible pgoff_t and since the type is
392: * unsigned we're using -1.
393: */
394: end = -1;
395: else
396: end = (lend + 1) >> PAGE_SHIFT;
397:
398: folio_batch_init(&fbatch);
399: index = start;
400: while (index < end && find_lock_entries(mapping, &index, end - 1,
401: &fbatch, indices)) {
402: truncate_folio_batch_exceptionals(mapping, &fbatch, indices);
403: for (i = 0; i < folio_batch_count(&fbatch); i++)
404: truncate_cleanup_folio(fbatch.folios[i]);
405: delete_from_page_cache_batch(mapping, &fbatch);
406: for (i = 0; i < folio_batch_count(&fbatch); i++)
407: folio_unlock(fbatch.folios[i]);
408: folio_batch_release(&fbatch);
409: cond_resched();
410: }
411:
412: same_folio = (lstart >> PAGE_SHIFT) == (lend >> PAGE_SHIFT);
413: folio = __filemap_get_folio(mapping, lstart >> PAGE_SHIFT, FGP_LOCK, 0);
414: if (!IS_ERR(folio)) {
415: same_folio = lend < folio_next_pos(folio);
416: if (!truncate_inode_partial_folio(folio, lstart, lend)) {
417: start = folio_next_index(folio);
418: if (same_folio)
419: end = folio->index;
420: }
421: folio_unlock(folio);
422: folio_put(folio);
423: folio = NULL;
424: }
425:
426: if (!same_folio) {
427: folio = __filemap_get_folio(mapping, lend >> PAGE_SHIFT,
428: FGP_LOCK, 0);
429: if (!IS_ERR(folio)) {
430: if (!truncate_inode_partial_folio(folio, lstart, lend))
431: end = folio->index;
432: folio_unlock(folio);
433: folio_put(folio);
434: }
435: }
436:
437: index = start;
438: while (index < end) {
439: cond_resched();
440: if (!find_get_entries(mapping, &index, end - 1, &fbatch,
441: indices)) {
442: /* If all gone from start onwards, we're done */
443: if (index == start)
444: break;
445: /* Otherwise restart to make sure all gone */
446: index = start;
447: continue;
448: }
449:
450: for (i = 0; i < folio_batch_count(&fbatch); i++) {
451: struct folio *folio = fbatch.folios[i];
452:
453: /* We rely upon deletion not changing folio->index */
454:
455: if (xa_is_value(folio))
456: continue;
457:
458: folio_lock(folio);
459: VM_BUG_ON_FOLIO(!folio_contains(folio, indices[i]), folio);
460: folio_wait_writeback(folio);
461: truncate_inode_folio(mapping, folio);
462: folio_unlock(folio);
463: }
464: truncate_folio_batch_exceptionals(mapping, &fbatch, indices);
465: folio_batch_release(&fbatch);
466: }
467: }
SourceFile:mm/truncate.c]
|
| 22/2 |
2026/05/15 11:27 |
llm |
expert |
1m
Model:
Tokens:
input: 37852
output: 41
thoughts: 582
Thoughts: **Analyzing RCU and Folio Batch Release in `truncate_inode_pages_range`**
Okay, so I'm looking at this `truncate_inode_pages_range` function, and specifically this loop. The thing that jumps out at me is `folio_batch_release`. It's called after the inner loop that calls `find_lock_entries`, but *also* outside of that function. My initial thought is: why isn't an RCU read lock held when `folio_batch_release` is called outside `find_lock_entries`? It seems like a potential race condition. This is because I expect tasks on the `blkd_tasks` list of the RCU node to be protected by that read lock.
Wait... could the RCU read lock *actually* be held? Where else could it be held? It's not immediately obvious from this snippet, so I need to step back and re-examine the context.
Let me check those stack traces again. I'm seeing tasks 25104 and 28178, both in `task_work_run`. That's intriguing. Could *that* function be holding the read lock? It's the most likely place outside of this specific function. To be certain, I need to check the definition of `task_work_run` and its relationship with RCU. That's the key to understanding this.
|
| 23/2 |
2026/05/15 11:29 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/task_work.c IncludeLines:true Name:task_work_run]
Results: map[SourceCode: 192: /**
193: * task_work_run - execute the works added by task_work_add()
194: *
195: * Flush the pending works. Should be used by the core kernel code.
196: * Called before the task returns to the user-mode or stops, or when
197: * it exits. In the latter case task_work_add() can no longer add the
198: * new work after task_work_run() returns.
199: */
200: void task_work_run(void)
201: {
202: struct task_struct *task = current;
203: struct callback_head *work, *head, *next;
204:
205: for (;;) {
206: /*
207: * work->func() can do task_work_add(), do not set
208: * work_exited unless the list is empty.
209: */
210: work = READ_ONCE(task->task_works);
211: do {
212: head = NULL;
213: if (!work) {
214: if (task->flags & PF_EXITING)
215: head = &work_exited;
216: else
217: break;
218: }
219: } while (!try_cmpxchg(&task->task_works, &work, head));
220:
221: if (!work)
222: break;
223: /*
224: * Synchronize with task_work_cancel_match(). It can not remove
225: * the first entry == work, cmpxchg(task_works) must fail.
226: * But it can remove another entry from the ->next list.
227: */
228: raw_spin_lock_irq(&task->pi_lock);
229: raw_spin_unlock_irq(&task->pi_lock);
230:
231: do {
232: next = work->next;
233: work->func(work);
234: work = next;
235: cond_resched();
236: } while (work);
237: }
238: }
SourceFile:kernel/task_work.c]
|
| 24/2 |
2026/05/15 11:29 |
llm |
expert |
0m
Model:
Tokens:
input: 39469
output: 39
thoughts: 227
Thoughts: **RCU Stall Detector Mystery**
Okay, so I'm digging into this RCU stall detector, and something feels off. I'm looking at the `task_work_run()` function, and I'm not seeing any `rcu_read_lock()` calls in there, which is as expected. However, the stall detector is reporting that tasks are blocked on the RCU node, right? But the stack traces aren't aligning.
My immediate thought is, could the stall detector be misinterpreting something? Is it *possible* that a task is flagged as blocked even if it *isn't* actually holding an `rcu_read_lock()`? That seems bizarre, but I need to rule it out. We know the stall detector uses the `blkd_tasks` list. If a task is preempted *while* it's holding `rcu_read_lock()`, it gets added to this list. When the task eventually calls `rcu_read_unlock()`, it then removes itself. So, if a task *is* on `blkd_tasks`, by definition, it *should* have called `rcu_read_lock()` and *not* yet called `rcu_read_unlock()`. That seems right.
But the stack trace I'm seeing has me completely stumped. The stack shows the task is in `folio_batch_release`, which is *outside* the `find_lock_entries` function. How can a task be holding `rcu_read_lock()` if it's in `folio_batch_release`? It doesn't make sense! This is just not adding up.
Alright, I need to backtrack, and perhaps rethink. Maybe something related to `__fput` is playing a role here. Let's start by looking at `__fput` in this context. Maybe `rcu_read_lock()` is being held there instead?
|
| 25/2 |
2026/05/15 11:29 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/file_table.c IncludeLines:true Name:__fput]
Results: map[SourceCode: 482: /* the real guts of fput() - releasing the last reference to file
483: */
484: static void __fput(struct file *file)
485: {
486: struct dentry *dentry = file->f_path.dentry;
487: struct vfsmount *mnt = file->f_path.mnt;
488: struct inode *inode = file->f_inode;
489: fmode_t mode = file->f_mode;
490:
491: if (unlikely(!(file->f_mode & FMODE_OPENED)))
492: goto out;
493:
494: might_sleep();
495:
496: fsnotify_close(file);
497: /*
498: * The function eventpoll_release() should be the first called
499: * in the file cleanup chain.
500: */
501: eventpoll_release(file);
502: locks_remove_file(file);
503:
504: security_file_release(file);
505: if (unlikely(file->f_flags & FASYNC)) {
506: if (file->f_op->fasync)
507: file->f_op->fasync(-1, file, 0);
508: }
509: if (file->f_op->release)
510: file->f_op->release(inode, file);
511: if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
512: !(mode & FMODE_PATH))) {
513: cdev_put(inode->i_cdev);
514: }
515: fops_put(file->f_op);
516: file_f_owner_release(file);
517: put_file_access(file);
518: dput(dentry);
519: if (unlikely(mode & FMODE_NEED_UNMOUNT))
520: dissolve_on_fput(mnt);
521: mntput(mnt);
522: out:
523: file_free(file);
524: }
SourceFile:fs/file_table.c]
|
| 26/2 |
2026/05/15 11:29 |
llm |
expert |
0m
Model:
Tokens:
input: 40827
output: 37
thoughts: 24
Thoughts: Okay, here's my interpretation of those thoughts, tailored for an expert audience, presented in the first person:
**Kernel Threading Quandary**
Hmm, so I'm looking at `__fput`. Something feels off... there's no `rcu_read_lock()` call in here. That's unusual, and it's making me reconsider the race conditions that I had assumed had been dealt with by other, higher level abstractions.
Then my mind drifts to `cleanup_mnt`. Could the same logic, or lack thereof, be present there? Or is it handled differently in the mount cleanup process? Need to dig into that and see how synchronization is handled around that part of the file system tear-down. This absence of RCU locking has me concerned about potential read-after-free scenarios, especially when dealing with concurrent file system operations. I need to understand the full picture of the data access here.
|
| 27/2 |
2026/05/15 11:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namespace.c IncludeLines:true Name:cleanup_mnt]
Results: map[SourceCode:1292: static void cleanup_mnt(struct mount *mnt)
1293: {
1294: struct hlist_node *p;
1295: struct mount *m;
1296: /*
1297: * The warning here probably indicates that somebody messed
1298: * up a mnt_want/drop_write() pair. If this happens, the
1299: * filesystem was probably unable to make r/w->r/o transitions.
1300: * The locking used to deal with mnt_count decrement provides barriers,
1301: * so mnt_get_writers() below is safe.
1302: */
1303: WARN_ON(mnt_get_writers(mnt));
1304: if (unlikely(mnt->mnt_pins.first))
1305: mnt_pin_kill(mnt);
1306: hlist_for_each_entry_safe(m, p, &mnt->mnt_stuck_children, mnt_umount) {
1307: hlist_del(&m->mnt_umount);
1308: mntput(&m->mnt);
1309: }
1310: fsnotify_vfsmount_delete(&mnt->mnt);
1311: dput(mnt->mnt.mnt_root);
1312: deactivate_super(mnt->mnt.mnt_sb);
1313: mnt_free_id(mnt);
1314: call_rcu(&mnt->mnt_rcu, delayed_free_vfsmnt);
1315: }
SourceFile:fs/namespace.c]
|
| 28/2 |
2026/05/15 11:30 |
llm |
expert |
2m
Model:
Tokens:
input: 41528
output: 47
thoughts: 2744
Thoughts: **RCU Stall Analysis: A Deep Dive**
Okay, so I'm looking at this RCU stall report and immediately something feels off. No `rcu_read_lock()` in `cleanup_mnt`? Hmm, how can tasks be on `blkd_tasks` then? That's the core of the issue. The clues begin to line up when I see the `rcu_preempt detected stalls` message. Specifically, tasks are blocked on level-0 rcu_node – now we're getting somewhere. Could they have been preempted while in `find_lock_entries`, which *does* hold `rcu_read_lock()`? That would add them to `blkd_tasks`. But if they resumed, finished, and called `rcu_read_unlock()`, wouldn't they be removed?
Ah, that's it! `rcu_read_unlock()` doesn't immediately remove them if there's nested RCU or if the grace period already ended. But if it's the outermost unlock, it *should* remove them. So, what about `rcu_read_unlock_special()`? That's the key. If preempted in a read-side critical section, its `.b.blocked` flag gets set. And when `rcu_read_unlock()` calls `rcu_read_unlock_special()`, the `rcu_node` lock is acquired, and the task *should* be removed. Could `rcu_read_unlock_special()` have been interrupted by a timer before removing the task? Possible, but the stack trace would show it. Here, it shows `folio_batch_release` instead.
Wait, what if the stalling task *isn't* the one holding the lock? What if it's just starved? And the report says `rcu: rcu_preempt kthread starved...` meaning *the* `rcu_preempt` kthread is starved. But why? CPU 1 is idle, but the kthread isn't scheduled? `->state=0x0` means `TASK_RUNNING`. It *should* be running! If it's running and CPU 1 is idle, why isn't it scheduled? It's in `pv_native_safe_halt`. So, the scheduler should pick the `rcu_preempt` kthread, and if it's not being picked, it may be on another CPU's runqueue, but `->cpu=1` means CPU 1's runqueue!
Why is CPU 1 idle with a runnable task? A known issue in virtualization or a scheduler bug? Let's check the stack trace: CPU 1 is in `default_idle`, but `rcu_preempt` is `TASK_RUNNING` *on* CPU 1. Is the kthread perhaps just woken up, but a wakeup IPI was lost or delayed? Or is the hypervisor (Google Compute Engine!) pausing the vCPU 1? "Hardware name: Google Google Compute Engine/Google Compute Engine." If the vCPU is paused, it won't process interrupts and the kthread won't run, but then I see that CPU 0 sent an NMI to CPU 1, and CPU 1 responded! So it's not paused. And if it's running, why is it in `default_idle` while `rcu_preempt` is runnable?
Wait a second, the `rcu_preempt` kthread state is `0x0` which is `TASK_RUNNING`. Could it actually be running on CPU 1? No, CPU 1 is in `default_idle`. Could it be on the runqueue but not scheduled due to a scheduler bug, or is it not even on CPU 1's runqueue? `->cpu=1` means `task_cpu(p)` is 1!
Now look at tasks 25104 and 28178: They are `state:R running task`, and Task 28178 *is* on CPU 1, *no*…`P28178/1:b..l` means PID 28178, and `1` is the CPU it was last running on, *but* if tasks 25104 and 28178 are both `state:R`, and `rcu_preempt` is `state:R`, and CPU 1 is idle…that means multiple tasks *are* runnable, but the CPU is idle! How? Throttled tasks, scheduler bug? Wait, what if the tasks *aren't* runnable, but their state *is* `R`? If a task gets preempted, its state remains `
|
| 29/2 |
2026/05/15 11:32 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/rcu/tree_plugin.h IncludeLines:true Name:rcu_read_unlock_special]
Results: map[SourceCode: 720: /*
721: * Handle special cases during rcu_read_unlock(), such as needing to
722: * notify RCU core processing or task having blocked during the RCU
723: * read-side critical section.
724: */
725: static void rcu_read_unlock_special(struct task_struct *t)
726: {
727: unsigned long flags;
728: bool irqs_were_disabled;
729: bool preempt_bh_were_disabled =
730: !!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
731:
732: /* NMI handlers cannot block and cannot safely manipulate state. */
733: if (in_nmi())
734: return;
735:
736: local_irq_save(flags);
737: irqs_were_disabled = irqs_disabled_flags(flags);
738: if (preempt_bh_were_disabled || irqs_were_disabled) {
739: bool needs_exp; // Expedited handling needed.
740: struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
741: struct rcu_node *rnp = rdp->mynode;
742:
743: needs_exp = rcu_unlock_needs_exp_handling(t, rdp, rnp, irqs_were_disabled);
744:
745: // Need to defer quiescent state until everything is enabled.
746: if (use_softirq && (in_hardirq() || (needs_exp && !irqs_were_disabled))) {
747: // Using softirq, safe to awaken, and either the
748: // wakeup is free or there is either an expedited
749: // GP in flight or a potential need to deboost.
750: if (rdp->defer_qs_pending != DEFER_QS_PENDING) {
751: rdp->defer_qs_pending = DEFER_QS_PENDING;
752: raise_softirq_irqoff(RCU_SOFTIRQ);
753: }
754: } else {
755: // Enabling BH or preempt does reschedule, so...
756: // Also if no expediting and no possible deboosting,
757: // slow is OK. Plus nohz_full CPUs eventually get
758: // tick enabled.
759: set_need_resched_current();
760: if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
761: needs_exp && rdp->defer_qs_pending != DEFER_QS_PENDING &&
762: cpu_online(rdp->cpu)) {
763: // Get scheduler to re-evaluate and call hooks.
764: // If !IRQ_WORK, FQS scan will eventually IPI.
765: rdp->defer_qs_pending = DEFER_QS_PENDING;
766: irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
767: }
768: }
769: local_irq_restore(flags);
770: return;
771: }
772: rcu_preempt_deferred_qs_irqrestore(t, flags);
773: }
SourceFile:kernel/rcu/tree_plugin.h]
|
| 30/2 |
2026/05/15 11:32 |
llm |
expert |
1m
Model:
Tokens:
input: 46329
output: 52
thoughts: 183
Thoughts: **RCU Quiescent State and Deferred States**
Okay, so `rcu_read_unlock_special` - I see, it defers the quiescent state if either `preempt_bh_were_disabled` or `irqs_were_disabled` are true. That's the key. Now, `find_lock_entries`, that shouldn't be disabling preempt or interrupts... right? Let me check... Hmm, but hang on, `rcu_read_unlock_special` *still* defers the quiescent state if `preempt_bh_were_disabled` is true.
The big question now is, is `preempt_bh_were_disabled` actually true when `rcu_read_unlock()` is called within `find_lock_entries()`? I have to trace the context of the call. No, that's right, `find_lock_entries` is called from `shmem_undo_range()`, and that runs in a normal process context. So that's unlikely to be the cause. But I can't be too sure!
Wait a minute, what if `rcu_read_unlock_special()` internally calls `rcu_preempt_deferred_qs_irqrestore()`? That's a crucial point to consider, I need to check the call stack in the problematic scenario.
|
| 31/2 |
2026/05/15 11:34 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/rcu/tree_plugin.h IncludeLines:true Name:rcu_preempt_deferred_qs_irqrestore]
Results: map[SourceCode: 472: /*
473: * Report deferred quiescent states. The deferral time can
474: * be quite short, for example, in the case of the call from
475: * rcu_read_unlock_special().
476: */
477: static notrace void
478: rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
479: {
480: bool empty_exp;
481: bool empty_norm;
482: bool empty_exp_now;
483: struct list_head *np;
484: bool drop_boost_mutex = false;
485: struct rcu_data *rdp;
486: struct rcu_node *rnp;
487: union rcu_special special;
488:
489: rdp = this_cpu_ptr(&rcu_data);
490: if (rdp->defer_qs_pending == DEFER_QS_PENDING)
491: rdp->defer_qs_pending = DEFER_QS_IDLE;
492:
493: /*
494: * If RCU core is waiting for this CPU to exit its critical section,
495: * report the fact that it has exited. Because irqs are disabled,
496: * t->rcu_read_unlock_special cannot change.
497: */
498: special = t->rcu_read_unlock_special;
499: if (!special.s && !rdp->cpu_no_qs.b.exp) {
500: local_irq_restore(flags);
501: return;
502: }
503: t->rcu_read_unlock_special.s = 0;
504: if (special.b.need_qs) {
505: if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {
506: rdp->cpu_no_qs.b.norm = false;
507: rcu_report_qs_rdp(rdp);
508: udelay(rcu_unlock_delay);
509: } else {
510: rcu_qs();
511: }
512: }
513:
514: /*
515: * Respond to a request by an expedited grace period for a
516: * quiescent state from this CPU. Note that requests from
517: * tasks are handled when removing the task from the
518: * blocked-tasks list below.
519: */
520: if (rdp->cpu_no_qs.b.exp)
521: rcu_report_exp_rdp(rdp);
522:
523: /* Clean up if blocked during RCU read-side critical section. */
524: if (special.b.blocked) {
525:
526: /*
527: * Remove this task from the list it blocked on. The task
528: * now remains queued on the rcu_node corresponding to the
529: * CPU it first blocked on, so there is no longer any need
530: * to loop. Retain a WARN_ON_ONCE() out of sheer paranoia.
531: */
532: rnp = t->rcu_blocked_node;
533: raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
534: WARN_ON_ONCE(rnp != t->rcu_blocked_node);
535: WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
536: empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
537: WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
538: (!empty_norm || rnp->qsmask));
539: empty_exp = sync_rcu_exp_done(rnp);
540: np = rcu_next_node_entry(t, rnp);
541: list_del_init(&t->rcu_node_entry);
542: t->rcu_blocked_node = NULL;
543: trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
544: rnp->gp_seq, t->pid);
545: if (&t->rcu_node_entry == rnp->gp_tasks)
546: WRITE_ONCE(rnp->gp_tasks, np);
547: if (&t->rcu_node_entry == rnp->exp_tasks)
548: WRITE_ONCE(rnp->exp_tasks, np);
549: if (IS_ENABLED(CONFIG_RCU_BOOST)) {
550: /* Snapshot ->boost_mtx ownership w/rnp->lock held. */
551: drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx.rtmutex) == t;
552: if (&t->rcu_node_entry == rnp->boost_tasks)
553: WRITE_ONCE(rnp->boost_tasks, np);
554: }
555:
556: /*
557: * If this was the last task on the current list, and if
558: * we aren't waiting on any CPUs, report the quiescent state.
559: * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
560: * so we must take a snapshot of the expedited state.
561: */
562: empty_exp_now = sync_rcu_exp_done(rnp);
563: if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
564: trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
565: rnp->gp_seq,
566: 0, rnp->qsmask,
567: rnp->level,
568: rnp->grplo,
569: rnp->grphi,
570: !!rnp->gp_tasks);
571: rcu_report_unblock_qs_rnp(rnp, flags);
572: } else {
573: raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
574: }
575:
576: /*
577: * If this was the last task on the expedited lists,
578: * then we need to report up the rcu_node hierarchy.
579: */
580: if (!empty_exp && empty_exp_now)
581: rcu_report_exp_rnp(rnp, true);
582:
583: /* Unboost if we were boosted. */
584: if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
585: rt_mutex_futex_unlock(&rnp->boost_mtx.rtmutex);
586: } else {
587: local_irq_restore(flags);
588: }
589: }
SourceFile:kernel/rcu/tree_plugin.h]
|
| 32/2 |
2026/05/15 11:34 |
llm |
expert |
0m
Model:
Tokens:
input: 48937
output: 43
thoughts: 414
Thoughts: **RCU Conundrum**
Okay, let's break this down. My initial observation is focused on `rcu_read_unlock_special`. The key point is that `rcu_preempt_deferred_qs_irqrestore` *only* gets called if `preempt_bh_were_disabled` is false. I've re-examined the code:
```c
if (preempt_bh_were_disabled || irqs_were_disabled) {
// ...
// defer quiescent state
return;
}
rcu_preempt_deferred_qs_irqrestore(t, flags);
```
If that condition is false – meaning, if we *don't* have preemption disabled for bottom halves (BHs) – then `rcu_preempt_deferred_qs_irqrestore` *does* get called. And critically, that function removes the task from `blkd_tasks`.
Now, here's where it gets interesting. `find_lock_entries` is triggered from `shmem_undo_range`, and that's running in a normal process context. So, `preempt_bh_were_disabled` is *definitely* false. Therefore, `rcu_read_unlock()` called inside `find_lock_entries` *should* immediately remove the task from `blkd_tasks`.
This presents a contradiction. If the task is immediately removed, it shouldn't be possible for it to still be on `blkd_tasks` while executing `folio_batch_release`. Unless... `folio_batch_release` is somehow called with preemption disabled for bottom halves? No, that doesn't seem right.
Aha! Perhaps `find_lock_entries` *isn't* the code that actually holds the RCU read lock in this scenario? Maybe the RCU read lock is held by another function that calls `find_lock_entries`. I need to figure out which code is actually holding the RCU read lock.
I checked `task_work_run`, but it doesn't hold `rcu_read_lock()`.
Now let's check `resume_user_mode_work`.
|
| 33/2 |
2026/05/15 11:35 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/entry/common.c IncludeLines:true Name:resume_user_mode_work]
Results: map[SourceCode: 28: /**
29: * resume_user_mode_work - Perform work before returning to user mode
30: * @regs: user-mode registers of @current task
31: *
32: * This is called when %TIF_NOTIFY_RESUME has been set. Now we are
33: * about to return to user mode, and the user state in @regs can be
34: * inspected or adjusted. The caller in arch code has cleared
35: * %TIF_NOTIFY_RESUME before the call. If the flag gets set again
36: * asynchronously, this will be called again before we return to
37: * user mode.
38: *
39: * Called without locks.
40: */
41: static inline void resume_user_mode_work(struct pt_regs *regs)
42: {
43: clear_thread_flag(TIF_NOTIFY_RESUME);
44: /*
45: * This barrier pairs with task_work_add()->set_notify_resume() after
46: * hlist_add_head(task->task_works);
47: */
48: smp_mb__after_atomic();
49: if (unlikely(task_work_pending(current)))
50: task_work_run();
51:
52: #ifdef CONFIG_KEYS_REQUEST_CACHE
53: if (unlikely(current->cached_requested_key)) {
54: key_put(current->cached_requested_key);
55: current->cached_requested_key = NULL;
56: }
57: #endif
58:
59: mem_cgroup_handle_over_high(GFP_KERNEL);
60: blkcg_maybe_throttle_current();
61:
62: rseq_handle_slowpath(regs);
63: }
SourceFile:include/linux/resume_user_mode.h]
|
| 34/2 |
2026/05/15 11:35 |
llm |
expert |
1m
Model:
Tokens:
input: 50428
output: 36
thoughts: 72
Thoughts: **Investigating a Potential Race Condition with RCU**
Okay, so I'm looking at this code and immediately I see that there's no explicit `rcu_read_lock()` in the area I was initially examining. That's the first thing I noticed and it's got me a little concerned. I have to think through where the read lock *could* be held.
Hang on a minute... what if `rcu_read_lock()` is actually being held by `__fput`? It's a common area for releasing resources, and I know those kinds of functions are often called in RCU scenarios. I'd better double-check that. Let me review the code for `__fput`... No, hold on, I've checked `__fput` and there's definitely no `rcu_read_lock()` there.
Alright, that's not it. Okay, where else could it be? Could it possibly be in `evict`? I know that function is involved in resource management related to file systems. So, the question remains: is the `rcu_read_lock()` in `evict`? I need to check `evict`...
|
| 35/2 |
2026/05/15 11:36 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/inode.c IncludeLines:true Name:evict]
Results: map[SourceCode: 805: /*
806: * Free the inode passed in, removing it from the lists it is still connected
807: * to. We remove any pages still attached to the inode and wait for any IO that
808: * is still in progress before finally destroying the inode.
809: *
810: * An inode must already be marked I_FREEING so that we avoid the inode being
811: * moved back onto lists if we race with other code that manipulates the lists
812: * (e.g. writeback_single_inode). The caller is responsible for setting this.
813: *
814: * An inode must already be removed from the LRU list before being evicted from
815: * the cache. This should occur atomically with setting the I_FREEING state
816: * flag, so no inodes here should ever be on the LRU when being evicted.
817: */
818: static void evict(struct inode *inode)
819: {
820: const struct super_operations *op = inode->i_sb->s_op;
821:
822: BUG_ON(!(inode_state_read_once(inode) & I_FREEING));
823: BUG_ON(!list_empty(&inode->i_lru));
824:
825: inode_io_list_del(inode);
826: inode_sb_list_del(inode);
827:
828: spin_lock(&inode->i_lock);
829: inode_wait_for_lru_isolating(inode);
830:
831: /*
832: * Wait for flusher thread to be done with the inode so that filesystem
833: * does not start destroying it while writeback is still running. Since
834: * the inode has I_FREEING set, flusher thread won't start new work on
835: * the inode. We just have to wait for running writeback to finish.
836: */
837: inode_wait_for_writeback(inode);
838: spin_unlock(&inode->i_lock);
839:
840: if (op->evict_inode) {
841: op->evict_inode(inode);
842: } else {
843: truncate_inode_pages_final(&inode->i_data);
844: clear_inode(inode);
845: }
846: if (S_ISCHR(inode->i_mode) && inode->i_cdev)
847: cd_forget(inode);
848:
849: remove_inode_hash(inode);
850:
851: /*
852: * Wake up waiters in __wait_on_freeing_inode().
853: *
854: * It is an invariant that any thread we need to wake up is already
855: * accounted for before remove_inode_hash() acquires ->i_lock -- both
856: * sides take the lock and sleep is aborted if the inode is found
857: * unhashed. Thus either the sleeper wins and goes off CPU, or removal
858: * wins and the sleeper aborts after testing with the lock.
859: *
860: * This also means we don't need any fences for the call below.
861: */
862: inode_wake_up_bit(inode, __I_NEW);
863: BUG_ON(inode_state_read_once(inode) != (I_FREEING | I_CLEAR));
864:
865: destroy_inode(inode);
866: }
SourceFile:fs/inode.c]
|
| 36/2 |
2026/05/15 11:36 |
llm |
expert |
1m
Model:
Tokens:
input: 51841
output: 42
thoughts: 27
Thoughts: Here's a summary of my immediate thought process, focusing on the key technical points:
**Analyzing Potential Locking Issues in File System Eviction**
Okay, I'm diving into this `evict` function, specifically wondering if there's a missing `rcu_read_lock()` call. That seems like a potential concurrency problem waiting to happen if we're dealing with shared memory. I need to understand the memory model and the data structures involved. RCU protection would be important to make sure no concurrent modifications are made when we are trying to evict an inode.
My next step is to examine `shmem_evict_inode`. That's where I need to focus my investigation. That's likely the key function here. I need to see how it handles concurrency and whether it provides the necessary synchronization to avoid race conditions during inode eviction. It may need to employ rcu protection.
|
| 37/2 |
2026/05/15 11:38 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/shmem.c IncludeLines:true Name:shmem_evict_inode]
Results: map[SourceCode:1397: static void shmem_evict_inode(struct inode *inode)
1398: {
1399: struct shmem_inode_info *info = SHMEM_I(inode);
1400: struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
1401: size_t freed = 0;
1402:
1403: if (shmem_mapping(inode->i_mapping)) {
1404: shmem_unacct_size(info->flags, inode->i_size);
1405: inode->i_size = 0;
1406: mapping_set_exiting(inode->i_mapping);
1407: shmem_truncate_range(inode, 0, (loff_t)-1);
1408: if (!list_empty(&info->shrinklist)) {
1409: spin_lock(&sbinfo->shrinklist_lock);
1410: if (!list_empty(&info->shrinklist)) {
1411: list_del_init(&info->shrinklist);
1412: sbinfo->shrinklist_len--;
1413: }
1414: spin_unlock(&sbinfo->shrinklist_lock);
1415: }
1416: while (!list_empty(&info->swaplist)) {
1417: /* Wait while shmem_unuse() is scanning this inode... */
1418: wait_var_event(&info->stop_eviction,
1419: !atomic_read(&info->stop_eviction));
1420: spin_lock(&shmem_swaplist_lock);
1421: /* ...but beware of the race if we peeked too early */
1422: if (!atomic_read(&info->stop_eviction))
1423: list_del_init(&info->swaplist);
1424: spin_unlock(&shmem_swaplist_lock);
1425: }
1426: }
1427:
1428: if (info->xattrs) {
1429: simple_xattrs_free(info->xattrs, sbinfo->max_inodes ? &freed : NULL);
1430: kfree(info->xattrs);
1431: }
1432: shmem_free_inode(inode->i_sb, freed);
1433: WARN_ON(inode->i_blocks);
1434: clear_inode(inode);
1435: #ifdef CONFIG_TMPFS_QUOTA
1436: dquot_free_inode(inode);
1437: dquot_drop(inode);
1438: #endif
1439: }
SourceFile:mm/shmem.c]
|
| 38/2 |
2026/05/15 11:38 |
llm |
expert |
1m
Model:
Tokens:
input: 52852
output: 39
thoughts: 982
Thoughts: **RCU Stall Investigation: A Deep Dive**
Okay, so I'm looking at this RCU stall report and something's clearly amiss. The stack trace indicates tasks are blocked on the RCU node, but my initial thought, "no `rcu_read_lock()` in `shmem_evict_inode`," doesn't seem to make sense. If there's no lock, why is the stall detector even flagging it?
Ah, I see a specific report: `Tasks blocked on level-0 rcu_node (CPUs 0-1): P25104/1:b..l P28178/1:b..l`. My initial theory, that tasks were preempted within `find_lock_entries` and were mistakenly left on `blkd_tasks` after resuming and calling `rcu_read_unlock()` but not being removed, seems plausible, but now I'm refining it. When *would* `rcu_read_unlock()` NOT remove a task from `blkd_tasks`? If `t->rcu_read_lock_nesting` is > 0! Could `rcu_read_lock()` have been called multiple times, and `find_lock_entries` only unlocked *one* of them? That would only happen if the caller of `shmem_undo_range` or `evict` held an `rcu_read_lock()`... but I've just checked those paths thoroughly: `evict`, `shmem_evict_inode`, `__fput`, `task_work_run`, `resume_user_mode_work`, `exit_to_user_mode_loop`, `syscall_exit_to_user_mode`, `do_syscall_64` - none of them are holding an RCU lock.
So, could there be a *leak*? Could `rcu_read_lock()` be held by some OTHER code that was called BEFORE `task_work_run`? If so, `t->rcu_read_lock_nesting` would be > 0. Then, `rcu_read_unlock()` in `find_lock_entries` would decrement, but not to zero, and the task would remain in the RCU critical section indefinitely! This would block the RCU grace period and explain the stall. Is there a known bug where `rcu_read_lock()` is leaked?
Or, perhaps `rcu_read_lock()` *is* held within `find_lock_entries`, the task is preempted, but hasn't reached `rcu_read_unlock()` yet? Hmm, but if it hasn't reached `rcu_read_unlock()`, how can the stack trace show it in `folio_batch_release`? Ah, that's because `folio_batch_release` is called inside `shmem_undo_range` *AFTER* `find_lock_entries` returns. Okay, if it has returned, it *must* have called `rcu_read_unlock()`.
Let's re-examine `find_lock_entries`:
```c
rcu_read_lock();
while ((folio = find_get_entry(&xas, end, XA_PRESENT))) {
// ...
if (!folio_batch_add(fbatch, folio))
break;
continue;
unlock:
folio_unlock(folio);
put:
folio_put(folio);
}
rcu_read_unlock();
```
Right, if `find_lock_entries` returns, it *has* called `rcu_read_unlock()`. And if it *has* called `rcu_read_unlock()`, and if `t->rcu_read_lock_nesting` becomes 0, it calls `rcu_read_unlock_special()`, which removes the task from `blkd_tasks`. Therefore, if the task is *still* on `blkd_tasks`, then `t->rcu_read_lock_nesting` must NOT be 0! Which *again* means `rcu_read_lock()` was leaked *somewhere*!
Wait... could `folio_batch_release` be called *inside* `find_lock_entries`? No, that's not the case. Okay, wait, what if `folio_put` *inside* `find_lock_entries` calls `folio_batch_release`? Now I'll check `folio_put`...
|
| 39/2 |
2026/05/15 11:39 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/mm.h IncludeLines:true Name:folio_put]
Results: map[SourceCode:2074: /**
2075: * folio_put - Decrement the reference count on a folio.
2076: * @folio: The folio.
2077: *
2078: * If the folio's reference count reaches zero, the memory will be
2079: * released back to the page allocator and may be used by another
2080: * allocation immediately. Do not access the memory or the struct folio
2081: * after calling folio_put() unless you can be sure that it wasn't the
2082: * last reference.
2083: *
2084: * Context: May be called in process or interrupt context, but not in NMI
2085: * context. May be called while holding a spinlock.
2086: */
2087: static inline void folio_put(struct folio *folio)
2088: {
2089: if (folio_put_testzero(folio))
2090: __folio_put(folio);
2091: }
SourceFile:include/linux/mm.h]
|
| 40/2 |
2026/05/15 11:39 |
llm |
expert |
1m
Model:
Tokens:
input: 55168
output: 38
thoughts: 24
Thoughts: Okay, here's my take on those thoughts, framed as an expert's internal monologue:
**Thinking Through Folio Management**
Alright, let's see... So, `__folio_put`... right, that's the function that's responsible for returning folios, usually after they've been used in some capacity. The question that immediately pops into my head is, does it directly invoke `free_unref_folios`? Or, and this is crucial for performance and resource management, does it batch those releases? I mean, directly freeing each folio one by one could be... suboptimal, especially under heavy load. A batching approach would allow for more efficient coalescing of freed memory and potentially fewer calls into the lower levels of the memory manager. It's a fundamental question of immediate versus delayed release and the potential for optimization. Need to dig into that code path and confirm the exact behavior. The devil is always in the details, isn't it?
|
| 41/2 |
2026/05/15 11:41 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/swap.c IncludeLines:true Name:__folio_put]
Results: map[SourceCode: 97: void __folio_put(struct folio *folio)
98: {
99: if (unlikely(folio_is_zone_device(folio))) {
100: free_zone_device_folio(folio);
101: return;
102: }
103:
104: if (folio_test_hugetlb(folio)) {
105: free_huge_folio(folio);
106: return;
107: }
108:
109: page_cache_release(folio);
110: folio_unqueue_deferred_split(folio);
111: mem_cgroup_uncharge(folio);
112: free_frozen_pages(&folio->page, folio_order(folio));
113: }
SourceFile:mm/swap.c]
|
| 42/2 |
2026/05/15 11:41 |
llm |
expert |
1m
Model:
Tokens:
input: 55689
output: 47
thoughts: 1509
Thoughts: **RCU Debugging: A Deep Dive into a Potential Leak**
Okay, let's break this down. My initial thought was that a `folio_batch_release` call was somehow being missed. The stack traces point to `shmem_undo_range` and `truncate_inode_pages_range`, indicating that the batch release is definitely happening *after* `find_lock_entries` has run. This is crucial because it means `rcu_read_unlock()` *must* have been called. If the task is still showing up on `blkd_tasks` *after* `rcu_read_unlock()`, that screams "RCU read lock leak"!
But is there another explanation? The traces are clear. They're definitely in the correct lines within `mm/shmem.c` (line 1149, right after `folio_batch_remove_exceptionals(&fbatch);`) and `mm/truncate.c` (line 408), respectively. This solidifies that the `folio_batch_release` call is indeed *after* `find_lock_entries` has finished.
Since both tasks are clearly outside `find_lock_entries` at the point of the batch release, they should *not* be holding an RCU read lock from there. So, the leak has to be elsewhere, even though they're still on `blkd_tasks`. Could `find_lock_entries` itself be the source? Let's re-examine that function. It looks clean, with a clear `rcu_read_unlock()` at the end of every execution path. No early returns, no gotos that skip the unlock.
If it's not `find_lock_entries`, where could it be? I've already ruled out `__fput`, `task_work_run`, and `exit_to_user_mode_loop`. What if the issue isn't a *leak* per se, but something related to how the task is handled after the unlock? Could `rcu_read_unlock_special()` be called, but fail to remove the task from `blkd_tasks`? Looking at the code for `rcu_read_unlock_special()`, there's that conditional block:
```c
if (preempt_bh_were_disabled || irqs_were_disabled) {
// ...
// defer quiescent state
return;
}
rcu_preempt_deferred_qs_irqrestore(t, flags);
```
Aha! If `preempt_bh_were_disabled` or `irqs_were_disabled` is true, it *defers* the quiescent state and *returns* without removing the task from `blkd_tasks`. Is this the key?
`preempt_bh_were_disabled` is dependent on `preempt_count()`. But `find_lock_entries` doesn't disable preemption or softirqs. However, `rcu_read_unlock_special` also checks `irqs_were_disabled`! The code does `local_irq_save(flags)`, meaning interrupts are temporarily disabled *within* `rcu_read_unlock_special`. The variable `irqs_were_disabled` holds the state *before* the `local_irq_save`. Now, do the interrupts disable in `find_lock_entries` before unlock? No, not there. But, could a task get preempted, and then resume with preemption or interrupts disabled? That should be impossible, as those counts should be 0 in normal process context.
Wait a second, what if the task *isn't* on `blkd_tasks` and the RCU stall detector is misleading me? No, the PIDs are unique. I have to check the logs to see what `b..l` means in the RCU printout.
The focus should be on the conditions under which an RCU task is *not* removed from `blkd_tasks` *after* a call to `rcu_read_unlock_special()`, especially in the face of disabled interrupts or preemption.
|
| 43/2 |
2026/05/15 11:42 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/rcu/tree_stall.h IncludeLines:true Name:rcu_print_task_stall]
Results: map[SourceCode: 320: /*
321: * Scan the current list of tasks blocked within RCU read-side critical
322: * sections, printing out the tid of each of the first few of them.
323: */
324: static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
325: __releases(rnp->lock)
326: {
327: int i = 0;
328: int ndetected = 0;
329: struct rcu_stall_chk_rdr rscr;
330: struct task_struct *t;
331: struct task_struct *ts[8];
332:
333: lockdep_assert_irqs_disabled();
334: if (!rcu_preempt_blocked_readers_cgp(rnp)) {
335: raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
336: return 0;
337: }
338: pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
339: rnp->level, rnp->grplo, rnp->grphi);
340: t = list_entry(rnp->gp_tasks->prev,
341: struct task_struct, rcu_node_entry);
342: list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
343: get_task_struct(t);
344: ts[i++] = t;
345: if (i >= ARRAY_SIZE(ts))
346: break;
347: }
348: raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
349: while (i) {
350: t = ts[--i];
351: if (task_call_func(t, check_slow_task, &rscr))
352: pr_cont(" P%d", t->pid);
353: else
354: pr_cont(" P%d/%d:%c%c%c%c",
355: t->pid, rscr.nesting,
356: ".b"[rscr.rs.b.blocked],
357: ".q"[rscr.rs.b.need_qs],
358: ".e"[rscr.rs.b.exp_hint],
359: ".l"[rscr.on_blkd_list]);
360: lockdep_assert_irqs_disabled();
361: put_task_struct(t);
362: ndetected++;
363: }
364: pr_cont("\n");
365: return ndetected;
366: }
SourceFile:kernel/rcu/tree_stall.h]
|
| 44/2 |
2026/05/15 11:42 |
llm |
expert |
2m
Model:
Tokens:
input: 58974
output: 41
thoughts: 679
Thoughts: **Investigating an Unexpected `rcu_read_lock()` State**
Okay, so I've got this clue: `P25104/1:b..l`. This tells me that the task with PID 25104 has `rscr.nesting` equal to 1, `rscr.rs.b.blocked` is true, `rscr.rs.b.need_qs` is false, `rscr.rs.b.exp_hint` is false, and `rscr.on_blkd_list` is true. Essentially, `t->rcu_read_lock_nesting` is 1! That means this task, right now, is holding *exactly one* `rcu_read_lock()`.
But hold on! The stack trace shows it's in `folio_batch_release`, and that function is *outside* the `find_lock_entries` code, where I would expect to see the lock being held. How is this possible? If `t->rcu_read_lock_nesting` is 1, it should be within the scope of the lock, and the call stack shows this to be in `folio_batch_release`!
Could `folio_batch_release` *itself* be responsible for holding the `rcu_read_lock()`? Let's take a closer look at the code:
```c
static inline void folio_batch_release(struct folio_batch *fbatch)
{
if (folio_batch_count(fbatch))
__folio_batch_release(fbatch);
}
```
```c
void __folio_batch_release(struct folio_batch *fbatch)
{
if (!fbatch->percpu_pvec_drained) {
lru_add_drain();
fbatch->percpu_pvec_drained = true;
}
folios_put(fbatch);
}
```
So it calls `lru_add_drain` and `folios_put`. Does *either* of those functions acquire `rcu_read_lock()`? Let's check `folios_put_refs`.
```c
void folios_put_refs(struct folio_batch *folios, unsigned int *refs)
{
// ...
for (i = 0, j = 0; i < folios->nr; i++) {
// ...
if (!folio_ref_sub_and_test(folio, nr_refs))
continue;
// ...
__page_cache_release(folio, &lruvec, &flags);
// ...
}
// ...
free_unref_folios(folios);
}
```
Okay, so `folios_put` calls `folios_put_refs`. Inside that, it calls `__page_cache_release` and `free_unref_folios`. I know that `__page_cache_release` and `free_unref_folios` are very unlikely to be the culprits, given what I know about them. So let's check `reset_page_owner`. And let's keep checking...
|
| 45/2 |
2026/05/15 11:44 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/page_owner.c IncludeLines:true Name:reset_page_owner]
Results: map[SourceCode: 22: static inline void reset_page_owner(struct page *page, unsigned short order)
23: {
24: if (static_branch_unlikely(&page_owner_inited))
25: __reset_page_owner(page, order);
26: }
SourceFile:include/linux/page_owner.h]
|
| 46/2 |
2026/05/15 11:44 |
llm |
expert |
1m
Model:
Tokens:
input: 60511
output: 42
thoughts: 0
|
| 47/2 |
2026/05/15 11:46 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/page_owner.c IncludeLines:true Name:__reset_page_owner]
Results: map[SourceCode: 298: void __reset_page_owner(struct page *page, unsigned short order)
299: {
300: struct page_ext *page_ext;
301: depot_stack_handle_t handle;
302: depot_stack_handle_t alloc_handle;
303: struct page_owner *page_owner;
304: u64 free_ts_nsec = local_clock();
305:
306: page_ext = page_ext_get(page);
307: if (unlikely(!page_ext))
308: return;
309:
310: page_owner = get_page_owner(page_ext);
311: alloc_handle = page_owner->handle;
312: page_ext_put(page_ext);
313:
314: /*
315: * Do not specify GFP_NOWAIT to make gfpflags_allow_spinning() == false
316: * to prevent issues in stack_depot_save().
317: * This is similar to alloc_pages_nolock() gfp flags, but only used
318: * to signal stack_depot to avoid spin_locks.
319: */
320: handle = save_stack(__GFP_NOWARN);
321: __update_page_owner_free_handle(page, handle, order, current->pid,
322: current->tgid, free_ts_nsec);
323:
324: if (alloc_handle != early_handle)
325: /*
326: * early_handle is being set as a handle for all those
327: * early allocated pages. See init_pages_in_zone().
328: * Since their refcount is not being incremented because
329: * the machinery is not ready yet, we cannot decrement
330: * their refcount either.
331: */
332: dec_stack_record_count(alloc_handle, 1 << order);
333: }
SourceFile:mm/page_owner.c]
|
| 48/2 |
2026/05/15 11:46 |
llm |
expert |
4m
Model:
Tokens:
input: 61158
output: 43
thoughts: 29
Thoughts: Okay, here's my take on those thoughts, framed as an expert's internal monologue:
**Investigating a Potential Race Condition**
Alright, that `page_ext_get` call has me a little concerned. I see it's calling `rcu_read_lock()` – interesting. My immediate thought is, let's drill down into that `page_ext_get` function. I need to understand why they're using RCU protection here. What kind of data is being accessed? Is there shared mutable state? Are they trying to protect against concurrent modification from, say, a page fault handler, or perhaps a different CPU accessing the page metadata? Understanding the *why* behind this lock is crucial to determine if this implementation is sound. This could be a critical area, potentially related to memory management, which I want to review now.
|
| 49/2 |
2026/05/15 11:51 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/page_ext.h IncludeLines:true Name:page_ext_get]
Results: map[SourceCode: 516: /**
517: * page_ext_get() - Get the extended information for a page.
518: * @page: The page we're interested in.
519: *
520: * Ensures that the page_ext will remain valid until page_ext_put()
521: * is called.
522: *
523: * Return: NULL if no page_ext exists for this page.
524: * Context: Any context. Caller may not sleep until they have called
525: * page_ext_put().
526: */
527: struct page_ext *page_ext_get(const struct page *page)
528: {
529: struct page_ext *page_ext;
530:
531: rcu_read_lock();
532: page_ext = lookup_page_ext(page);
533: if (!page_ext) {
534: rcu_read_unlock();
535: return NULL;
536: }
537:
538: return page_ext;
539: }
SourceFile:mm/page_ext.c]
|
| 50/2 |
2026/05/15 11:51 |
llm |
expert |
2m
Model:
Tokens:
input: 61766
output: 46
thoughts: 244
Thoughts: **Analyzing the Page Owner Reset Sequence**
Okay, so I'm looking at this `__reset_page_owner` function. The first thing that jumps out is that `page_ext_get` uses `rcu_read_lock()` and `page_ext_put` uses `rcu_read_unlock()`. Good, that's expected. We're using RCU properly for concurrent access to page extension metadata.
Let's trace the logic. We grab the page extension, get the owner, store the allocation handle, and then... *aha!* `page_ext_put` is called *before* we call `save_stack` and `__update_page_owner_free_handle`! That means the `rcu_read_unlock()` is happening *before* we capture the relevant information for the stack trace and update the freed allocation handle.
This seems... potentially problematic. We could potentially have an issue if something were to happen in between the calls. What about within `__update_page_owner_free_handle`? Let's check there...
|
| 51/2 |
2026/05/15 11:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/page_owner.c IncludeLines:true Name:__update_page_owner_free_handle]
Results: map[SourceCode: 273: static inline void __update_page_owner_free_handle(struct page *page,
274: depot_stack_handle_t handle,
275: unsigned short order,
276: pid_t pid, pid_t tgid,
277: u64 free_ts_nsec)
278: {
279: struct page_ext_iter iter;
280: struct page_ext *page_ext;
281: struct page_owner *page_owner;
282:
283: rcu_read_lock();
284: for_each_page_ext(page, 1 << order, page_ext, iter) {
285: page_owner = get_page_owner(page_ext);
286: /* Only __reset_page_owner() wants to clear the bit */
287: if (handle) {
288: __clear_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags);
289: page_owner->free_handle = handle;
290: }
291: page_owner->free_ts_nsec = free_ts_nsec;
292: page_owner->free_pid = current->pid;
293: page_owner->free_tgid = current->tgid;
294: }
295: rcu_read_unlock();
296: }
SourceFile:mm/page_owner.c]
|
| 52/2 |
2026/05/15 11:53 |
llm |
expert |
0m
Model:
Tokens:
input: 62718
output: 49
thoughts: 614
Thoughts: **Analyzing the Page Owner Logic**
Okay, so I'm digging into this page owner issue. The first thing that jumps out is that `__update_page_owner_free_handle` is calling `rcu_read_lock()`. That's important. I see task 25104 is currently executing inside `__update_page_owner_free_handle`. The call stack confirms this, showing the execution flow through `__nr_to_section`, `__pfn_to_section`, `lookup_page_ext`, then finally `__update_page_owner_free_handle`, specifically at line 284: `for_each_page_ext(page, 1 << order, page_ext, iter)`. This `for_each_page_ext` loop, and therefore *the entire function* is within the RCU read lock. So, at the point of the crash, task 25104 **is** holding the `rcu_read_lock`.
Now, I need to understand task 28178. Its call stack shows it's executing `__reset_page_owner` where `handle = save_stack(__GFP_NOWARN);` is called at line 320. Critically, this `save_stack` call is *outside* the `rcu_read_lock()` context. So, if this is truly outside the RCU lock, why is it conflicting? It's on `blkd_tasks` and that doesn't make sense if it's outside. This suggests a problem in the timing. I'm thinking about whether `save_stack` might internally call `rcu_read_lock()` itself, even though the surrounding function isn't.
Time to trace `save_stack`: `save_stack` -> `stack_trace_save` -> `arch_stack_walk` -> `unwind_next_frame`. I need to investigate the implementation of those functions, to see if they are doing something with the RCU locks I didn't see. Is there a hidden `rcu_read_lock()` call in that sequence? That's likely where the problem lies.
|
| 53/2 |
2026/05/15 11:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:arch/x86/kernel/unwind_orc.c IncludeLines:true Name:unwind_next_frame]
Results: map[SourceCode: 484: bool unwind_next_frame(struct unwind_state *state)
485: {
486: unsigned long ip_p, sp, tmp, orig_ip = state->ip, prev_sp = state->sp;
487: enum stack_type prev_type = state->stack_info.type;
488: struct orc_entry *orc;
489: bool indirect = false;
490:
491: if (unwind_done(state))
492: return false;
493:
494: /* Don't let modules unload while we're reading their ORC data. */
495: guard(rcu)();
496:
497: /* End-of-stack check for user tasks: */
498: if (state->regs && user_mode(state->regs))
499: goto the_end;
500:
501: /*
502: * Find the orc_entry associated with the text address.
503: *
504: * For a call frame (as opposed to a signal frame), state->ip points to
505: * the instruction after the call. That instruction's stack layout
506: * could be different from the call instruction's layout, for example
507: * if the call was to a noreturn function. So get the ORC data for the
508: * call instruction itself.
509: */
510: orc = orc_find(state->signal ? state->ip : state->ip - 1);
511: if (!orc) {
512: /*
513: * As a fallback, try to assume this code uses a frame pointer.
514: * This is just a guess, so the rest of the unwind is no longer
515: * considered reliable.
516: */
517: orc = &orc_fp_entry;
518: state->error = true;
519: } else {
520: if (orc->type == ORC_TYPE_UNDEFINED)
521: goto err;
522:
523: if (orc->type == ORC_TYPE_END_OF_STACK)
524: goto the_end;
525: }
526:
527: state->signal = orc->signal;
528:
529: /* Find the previous frame's stack: */
530: switch (orc->sp_reg) {
531: case ORC_REG_SP:
532: sp = state->sp + orc->sp_offset;
533: break;
534:
535: case ORC_REG_BP:
536: sp = state->bp + orc->sp_offset;
537: break;
538:
539: case ORC_REG_SP_INDIRECT:
540: sp = state->sp;
541: indirect = true;
542: break;
543:
544: case ORC_REG_BP_INDIRECT:
545: sp = state->bp + orc->sp_offset;
546: indirect = true;
547: break;
548:
549: /*
550: * Any of the below registers may temporarily hold the stack pointer,
551: * typically during a DRAP stack realignment sequence or some other
552: * stack swizzle.
553: */
554:
555: case ORC_REG_AX:
556: if (!get_reg(state, offsetof(struct pt_regs, ax), &sp)) {
557: orc_warn_current("missing AX value at %pB\n",
558: (void *)state->ip);
559: goto err;
560: }
561: break;
562:
563: case ORC_REG_DX:
564: if (!get_reg(state, offsetof(struct pt_regs, dx), &sp)) {
565: orc_warn_current("missing DX value at %pB\n",
566: (void *)state->ip);
567: goto err;
568: }
569: break;
570:
571: case ORC_REG_DI:
572: if (!get_reg(state, offsetof(struct pt_regs, di), &sp)) {
573: orc_warn_current("missing RDI value at %pB\n",
574: (void *)state->ip);
575: goto err;
576: }
577: break;
578:
579: case ORC_REG_R10:
580: if (!get_reg(state, offsetof(struct pt_regs, r10), &sp)) {
581: orc_warn_current("missing R10 value at %pB\n",
582: (void *)state->ip);
583: goto err;
584: }
585: break;
586:
587: case ORC_REG_R13:
588: if (!get_reg(state, offsetof(struct pt_regs, r13), &sp)) {
589: orc_warn_current("missing R13 value at %pB\n",
590: (void *)state->ip);
591: goto err;
592: }
593: break;
594:
595: default:
596: orc_warn("unknown SP base reg %d at %pB\n",
597: orc->sp_reg, (void *)state->ip);
598: goto err;
599: }
600:
601: if (indirect) {
602: if (!deref_stack_reg(state, sp, &sp))
603: goto err;
604:
605: if (orc->sp_reg == ORC_REG_SP_INDIRECT)
606: sp += orc->sp_offset;
607: }
608:
609: /* Find IP, SP and possibly regs: */
610: switch (orc->type) {
611: case ORC_TYPE_CALL:
612: ip_p = sp - sizeof(long);
613:
614: if (!deref_stack_reg(state, ip_p, &state->ip))
615: goto err;
616:
617: state->ip = unwind_recover_ret_addr(state, state->ip,
618: (unsigned long *)ip_p);
619: state->sp = sp;
620: state->regs = NULL;
621: state->prev_regs = NULL;
622: break;
623:
624: case ORC_TYPE_REGS:
625: if (!deref_stack_regs(state, sp, &state->ip, &state->sp)) {
626: orc_warn_current("can't access registers at %pB\n",
627: (void *)orig_ip);
628: goto err;
629: }
630: /*
631: * There is a small chance to interrupt at the entry of
632: * arch_rethook_trampoline() where the ORC info doesn't exist.
633: * That point is right after the RET to arch_rethook_trampoline()
634: * which was modified return address.
635: * At that point, the @addr_p of the unwind_recover_rethook()
636: * (this has to point the address of the stack entry storing
637: * the modified return address) must be "SP - (a stack entry)"
638: * because SP is incremented by the RET.
639: */
640: state->ip = unwind_recover_rethook(state, state->ip,
641: (unsigned long *)(state->sp - sizeof(long)));
642: state->regs = (struct pt_regs *)sp;
643: state->prev_regs = NULL;
644: state->full_regs = true;
645: break;
646:
647: case ORC_TYPE_REGS_PARTIAL:
648: if (!deref_stack_iret_regs(state, sp, &state->ip, &state->sp)) {
649: orc_warn_current("can't access iret registers at %pB\n",
650: (void *)orig_ip);
651: goto err;
652: }
653: /* See ORC_TYPE_REGS case comment. */
654: state->ip = unwind_recover_rethook(state, state->ip,
655: (unsigned long *)(state->sp - sizeof(long)));
656:
657: if (state->full_regs)
658: state->prev_regs = state->regs;
659: state->regs = (void *)sp - IRET_FRAME_OFFSET;
660: state->full_regs = false;
661: break;
662:
663: default:
664: orc_warn("unknown .orc_unwind entry type %d at %pB\n",
665: orc->type, (void *)orig_ip);
666: goto err;
667: }
668:
669: /* Find BP: */
670: switch (orc->bp_reg) {
671: case ORC_REG_UNDEFINED:
672: if (get_reg(state, offsetof(struct pt_regs, bp), &tmp))
673: state->bp = tmp;
674: break;
675:
676: case ORC_REG_PREV_SP:
677: if (!deref_stack_reg(state, sp + orc->bp_offset, &state->bp))
678: goto err;
679: break;
680:
681: case ORC_REG_BP:
682: if (!deref_stack_reg(state, state->bp + orc->bp_offset, &state->bp))
683: goto err;
684: break;
685:
686: default:
687: orc_warn("unknown BP base reg %d for ip %pB\n",
688: orc->bp_reg, (void *)orig_ip);
689: goto err;
690: }
691:
692: /* Prevent a recursive loop due to bad ORC data: */
693: if (state->stack_info.type == prev_type &&
694: on_stack(&state->stack_info, (void *)state->sp, sizeof(long)) &&
695: state->sp <= prev_sp) {
696: orc_warn_current("stack going in the wrong direction? at %pB\n",
697: (void *)orig_ip);
698: goto err;
699: }
700:
701: return true;
702:
703: err:
704: state->error = true;
705:
706: the_end:
707: state->stack_info.type = STACK_TYPE_UNKNOWN;
708: return false;
709: }
SourceFile:arch/x86/kernel/unwind_orc.c]
|
| 54/2 |
2026/05/15 11:53 |
llm |
expert |
1m
Model:
Tokens:
input: 67209
output: 66
thoughts: 4989
Thoughts: **RCU Stall Analysis: A Deep Dive**
Okay, so I've got this RCU stall reported, and my mind is racing. Initially, I saw `unwind_next_frame` calling `guard(rcu)();`, and I started connecting the dots, thinking that `rcu_read_lock()` might be held for an extended period, leading to a blocked grace period. I was focusing on how `unwind_next_frame` gets called repeatedly within `arch_stack_walk`, which runs in a loop, increasing the chances of preemption and adding tasks to `blkd_tasks`. I was concerned that this could starve the grace period for an extended amount of time. Then, I realized that `guard(rcu)()` is a scope, and the lock is released at the end of the `unwind_next_frame`, so it wouldn't block RCU for a long time. So that led me to think the core issue was elsewhere.
Then, my attention shifted to the provided stack trace, particularly the `__update_page_owner_free_handle` function, along with tasks 25104 and 28178, and the page freeing and stack unwinding. I saw a loop that could potentially take a long time, especially when dealing with large page orders. However, I calculated that the loop itself isn't the bottleneck and wouldn't cause the stall. I began to consider that since `unwind_next_frame` (called when we are freeing pages) is slow, it might cause the grace period to stall.
The error message, "rcu: rcu_preempt kthread starved for 10598 jiffies," clarified the problem: the `rcu_preempt` kthread isn't getting CPU time, it's not the tasks that hold the lock but the kthread not being scheduled. I realized it's a starvation issue, which, if true, means it's not a direct consequence of the RCU read-side critical sections themselves.
I thought about the scenario on an idle CPU; it can't finish the RCU grace period. This is when the stall detector runs, and we see tasks 25104 and 28178. However, because the RCU kthread is starved, that's not their fault! The tasks are freeing a lot of pages, which is why they happen to be in the code when the stall happens. This situation is the consequence of the code that's already in the read-side critical section.
So, I shifted my focus to why the kthread might be starved. Then, I remembered that this is an expected behavior with softlockups, or heavy page faulting. Then, I went to the hypervisor bug. This can be caused because the VMs might be running in an environment where the hypervisor is causing a scheduling issue. With the hypervisor not sending an interrupt on time, this can cause the system to stall. I had a hypothesis that this can also be the consequence of running slow debugging features like `page_owner`.
I started to consider if the tasks could somehow be *causing* the starvation, by taking up CPU time, but the kthread has a high priority (if `RCU_BOOST` is enabled). Then, it became clear that the problem is not about CPU usage. The root cause is the `rcu_preempt` kthread not being runnable or runnable but waiting for a timer, because of scheduling issues.
Now, the analysis turned to the key aspect: the build environment. The instructions stated "production build w/o debugging tools enabled." I realized the primary culprit: `page_owner`! That's the debugging feature causing the incredibly slow stack unwinding for each freed page. This is likely the root of the problem.
In a production build, with `page_owner` disabled, freeing pages is fast. The stall simply won't happen. The reported stall is a false positive due to the debugging overhead, combined with some scheduling or hypervisor problem. This is not a security issue in a production setting.
To be sure, I double-checked the code for potential infinite loops in `shmem_undo_range` and `truncate_inode_pages_range`, focusing on the `find_lock_entries` function and its interactions with page cache operations. I confirmed that the index gets updated even if pages are not completely removed from the cache, so we are not going to be in a loop.
So the conclusion is: The RCU stall is a performance issue tied to `page_owner`.
I will mark it as Not Exploitable, and no Denial of Service.
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| 55/2 |
2026/05/15 11:55 |
tool |
set-results |
0m
Args: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:true VMHostTrigger:false]
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:true VMHostTrigger:false]
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| 56/2 |
2026/05/15 11:55 |
llm |
expert |
0m
Model:
Tokens:
input: 73317
output: 547
thoughts: 127
Thoughts: **Finalizing the Response**
Okay, the results are in, and now it's time to craft the definitive response. The core of this explanation needs to highlight the root cause: this RCU stall is directly attributable to the excessive overhead introduced by the `page_owner` debugging feature. Specifically, the problem manifests when a large number of pages are being freed – think large file truncation or eviction. It's really that `page_owner` tracking, while incredibly useful for debugging, becomes the bottleneck here.
Crucially, I need to emphasize the non-impact in production environments. We can say it's not a concern, because `page_owner` is *not* enabled in production builds. This means no security implications. There's no risk of Denial of Service (DoS) and no opportunity for privilege escalation. Yes, unprivileged users *can* trigger the code path that leads to this issue (file deletion, file truncation), but the crucial piece is that it won't manifest as a bug in a production environment. So, we're looking at a debugging artifact, not a security flaw.
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| 57/1 |
2026/05/15 11:56 |
action |
format-explanation |
0m
Results: map[Explanation:Based on the analysis of the stack traces and the kernel source code, here is
the assessment of the bug:
**Root Cause Analysis:**
The bug report indicates an RCU stall where tasks are blocked on an RCU node,
preventing the RCU grace period kthread from completing its work. The stack
traces for the blocked tasks (`syz-executor`) show that they are in the process
of freeing a large number of pages during file eviction/truncation
(`shmem_undo_range` and `truncate_inode_pages_range`).
Specifically, the tasks are deep inside the `page_owner` tracking code
(`__reset_page_owner`, `save_stack`, `unwind_next_frame`, and
`__update_page_owner_free_handle`). `page_owner` is a kernel debugging feature
that records the allocation and freeing stack traces for every single page in
the system. Stack unwinding (`unwind_next_frame`) is a computationally expensive
and slow operation.
When a process deletes or truncates a very large file, it frees thousands or
millions of pages in a loop. With `page_owner` enabled, the kernel has to unwind
and save the stack trace for every single page being freed. This massive
overhead keeps the CPUs busy for an extended period (over 10.5 seconds in this
report), which starves the `rcu_preempt` kthread and triggers the RCU stall
detector.
**Security Impact:**
The `page_owner` feature is strictly a debugging and memory profiling tool
(enabled via `CONFIG_PAGE_OWNER=y` and the `page_owner=on` boot parameter). It
is **not enabled in production kernel builds** due to its severe performance
overhead.
In a standard production environment without `page_owner` and other heavy
debugging tools (like KASAN), freeing pages is an extremely fast operation. The
loops in `shmem_undo_range` and `truncate_inode_pages_range` will complete
quickly and will not cause an RCU stall or CPU lockup. There are no infinite
loops in these functions, as the page cache iterators correctly advance their
indices regardless of whether the pages are immediately freed.
Therefore, this is a false positive caused by the artificial overhead of
debugging tools. It does not represent a Denial of Service (DoS) or an
exploitable vulnerability in production systems.
**Trigger Vectors:**
While the bug itself is not present in production, the underlying code path
(creating and deleting/truncating large files) is reachable by any unprivileged
user, including those inside user namespaces or running within a VM guest.]
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