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BUG: KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic

write to 0xffff888122a50c1c of 4 bytes by task 17359 on cpu 1:
 __fat_write_inode+0x236/0x530 fs/fat/inode.c:-1
 fat_write_inode+0xbf/0xd0 fs/fat/inode.c:925
 write_inode fs/fs-writeback.c:1582 [inline]
 __writeback_single_inode+0x315/0x720 fs/fs-writeback.c:1825
 writeback_single_inode+0x16d/0x3f0 fs/fs-writeback.c:1881
 sync_inode_metadata+0x6e/0xa0 fs/fs-writeback.c:2992
 writeback_inode fs/fat/inode.c:1875 [inline]
 fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1895
 fat_truncate_blocks+0x528/0x5a0 fs/fat/file.c:396
 fat_setattr+0x78e/0x870 fs/fat/file.c:552
 notify_change+0x872/0x8f0 fs/attr.c:556
 do_truncate+0x14f/0x190 fs/open.c:68
 handle_truncate fs/namei.c:4285 [inline]
 do_open fs/namei.c:4681 [inline]
 path_openat+0x1c97/0x2050 fs/namei.c:4836
 do_file_open+0x16c/0x290 fs/namei.c:4865
 do_sys_openat2+0x94/0x130 fs/open.c:1366
 do_sys_open fs/open.c:1372 [inline]
 __do_sys_openat fs/open.c:1388 [inline]
 __se_sys_openat fs/open.c:1383 [inline]
 __x64_sys_openat+0xf2/0x120 fs/open.c:1383
 x64_sys_call+0x1e39/0x3020 arch/x86/include/generated/asm/syscalls_64.h:258
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888122a50c00 of 1024 bytes by task 17154 on cpu 0:
 memcpy_from_iter lib/iov_iter.c:85 [inline]
 iterate_bvec include/linux/iov_iter.h:123 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:306 [inline]
 iterate_and_advance include/linux/iov_iter.h:330 [inline]
 __copy_from_iter lib/iov_iter.c:261 [inline]
 copy_folio_from_iter_atomic+0x75f/0x1170 lib/iov_iter.c:491
 generic_perform_write+0x2c1/0x490 mm/filemap.c:4342
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3502
 lo_rw_aio+0x67d/0x730 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1925 [inline]
 loop_process_work+0x56c/0xac0 drivers/block/loop.c:1960
 loop_workfn+0x31/0x40 drivers/block/loop.c:1984
 process_one_work kernel/workqueue.c:3276 [inline]
 process_scheduled_works+0x513/0xa10 kernel/workqueue.c:3359
 worker_thread+0x58a/0x780 kernel/workqueue.c:3440
 kthread+0x22a/0x280 kernel/kthread.c:436
 ret_from_fork+0x150/0x360 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 17154 Comm: kworker/u8:31 Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Workqueue: loop8 loop_workfn
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