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BUG: KCSAN: data-race in copy_page_from_iter_atomic / fat16_ent_put

write to 0xffff888138b586b6 of 2 bytes by task 26948 on cpu 0:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
 fat_ent_write+0x69/0xe0 fs/fat/fatent.c:417
 fat_chain_add+0x15b/0x3f0 fs/fat/misc.c:136
 fat_add_cluster fs/fat/inode.c:112 [inline]
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x46c/0x5e0 fs/fat/inode.c:189
 __block_write_begin_int+0x419/0xf70 fs/buffer.c:2151
 block_write_begin fs/buffer.c:2262 [inline]
 cont_write_begin+0x5a3/0x8e0 fs/buffer.c:2601
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 cont_expand_zero fs/buffer.c:2528 [inline]
 cont_write_begin+0x198/0x8e0 fs/buffer.c:2591
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 generic_cont_expand_simple+0xad/0x150 fs/buffer.c:2492
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
 fat_setattr+0x2a5/0x8a0 fs/fat/file.c:513
 notify_change+0x806/0x890 fs/attr.c:552
 do_truncate fs/open.c:65 [inline]
 do_ftruncate+0x327/0x430 fs/open.c:192
 do_sys_ftruncate fs/open.c:207 [inline]
 __do_sys_ftruncate fs/open.c:212 [inline]
 __se_sys_ftruncate fs/open.c:210 [inline]
 __x64_sys_ftruncate+0x68/0xc0 fs/open.c:210
 x64_sys_call+0xd65/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:78
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd0/0x1a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888138b58600 of 512 bytes by task 8336 on cpu 1:
 memcpy_from_iter lib/iov_iter.c:73 [inline]
 iterate_bvec include/linux/iov_iter.h:123 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:304 [inline]
 iterate_and_advance include/linux/iov_iter.h:328 [inline]
 __copy_from_iter lib/iov_iter.c:249 [inline]
 copy_page_from_iter_atomic+0x77f/0xff0 lib/iov_iter.c:483
 copy_folio_from_iter_atomic include/linux/uio.h:210 [inline]
 generic_perform_write+0x2c2/0x490 mm/filemap.c:4121
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3464
 lo_rw_aio+0x5f7/0x7c0 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1866 [inline]
 loop_process_work+0x52d/0xa60 drivers/block/loop.c:1901
 loop_workfn+0x31/0x40 drivers/block/loop.c:1925
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0x4cb/0x9d0 kernel/workqueue.c:3319
 worker_thread+0x582/0x770 kernel/workqueue.c:3400
 kthread+0x486/0x510 kernel/kthread.c:464
 ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:153
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 8336 Comm: kworker/u8:11 Tainted: G        W           6.15.0-rc4-syzkaller-00291-g2a239ffbebb5 #0 PREEMPT(voluntary) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/19/2025
Workqueue: loop9 loop_workfn
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