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BUG: KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic

write to 0xffff88815f3548bc of 4 bytes by task 3130 on cpu 0:
 __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_write_failed fs/fat/inode.c:219 [inline]
 fat_write_end+0xba/0x160 fs/fat/inode.c:247
 generic_perform_write+0x311/0x490 mm/filemap.c:4345
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4441
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4467
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x5a6/0x9f0 fs/read_write.c:688
 ksys_write+0xdc/0x1a0 fs/read_write.c:740
 __do_sys_write fs/read_write.c:751 [inline]
 __se_sys_write fs/read_write.c:748 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:748
 x64_sys_call+0x27e1/0x3020 arch/x86/include/generated/asm/syscalls_64.h:2
 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 0xffff88815f354800 of 2048 bytes by task 426 on cpu 1:
 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: 1 UID: 0 PID: 426 Comm: kworker/u8:22 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Workqueue: loop4 loop_workfn
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