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BUG: KCSAN: data-race in ext4_nonda_switch / percpu_counter_add_batch

read to 0xffff8881264680e0 of 8 bytes by task 4740 on cpu 1:
 ext4_nonda_switch+0x3d/0x140 fs/ext4/inode.c:2988
 ext4_page_mkwrite+0x748/0xce0 fs/ext4/inode.c:6215
 do_page_mkwrite+0x100/0x210 mm/memory.c:2208
 do_shared_fault mm/memory.c:3554 [inline]
 do_fault mm/memory.c:3622 [inline]
 handle_pte_fault mm/memory.c:3849 [inline]
 __handle_mm_fault+0x2099/0x2cb0 mm/memory.c:3973
 handle_mm_fault+0x21b/0x530 mm/memory.c:4010
 do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
 __do_page_fault+0x3fb/0x9e0 arch/x86/mm/fault.c:1506
 do_page_fault+0x54/0x233 arch/x86/mm/fault.c:1530
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1202

write to 0xffff8881264680e0 of 8 bytes by task 4742 on cpu 0:
 percpu_counter_add_batch+0xca/0x150 lib/percpu_counter.c:91
 percpu_counter_add include/linux/percpu_counter.h:55 [inline]
 ext4_free_blocks+0xb4d/0x17e0 fs/ext4/mballoc.c:4928
 ext4_remove_blocks fs/ext4/extents.c:2678 [inline]
 ext4_ext_rm_leaf fs/ext4/extents.c:2831 [inline]
 ext4_ext_remove_space+0x1841/0x32c0 fs/ext4/extents.c:3076
 ext4_ext_truncate+0x120/0x130 fs/ext4/extents.c:4599
 ext4_truncate+0x5a2/0x990 fs/ext4/inode.c:4511
 ext4_setattr+0xfb4/0x1620 fs/ext4/inode.c:5665
 notify_change+0x7e1/0xaa0 fs/attr.c:337
 do_truncate+0xfb/0x180 fs/open.c:64
 handle_truncate fs/namei.c:2998 [inline]
 do_last fs/namei.c:3414 [inline]
 path_openat+0x2f4a/0x36e0 fs/namei.c:3525
 do_filp_open+0x11e/0x1b0 fs/namei.c:3555
 do_sys_open+0x3b3/0x4f0 fs/open.c:1097
 ksys_open include/linux/syscalls.h:1385 [inline]
 __do_sys_creat fs/open.c:1155 [inline]
 __se_sys_creat fs/open.c:1153 [inline]
 __x64_sys_creat+0x45/0x60 fs/open.c:1153
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 4742 Comm: syz-executor.1 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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