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BUG: KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic

write to 0xffff88811c5b1c1c of 4 bytes by task 7019 on cpu 1:
 __fat_write_inode+0x236/0x520 fs/fat/inode.c:-1
 fat_write_inode+0xbf/0xd0 fs/fat/inode.c:924
 write_inode fs/fs-writeback.c:1564 [inline]
 __writeback_single_inode+0x31c/0x7c0 fs/fs-writeback.c:1784
 writeback_single_inode+0x16d/0x3f0 fs/fs-writeback.c:1840
 sync_inode_metadata+0x5b/0x90 fs/fs-writeback.c:2923
 writeback_inode fs/fat/inode.c:1870 [inline]
 fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1890
 fat_truncate_blocks+0x4d8/0x550 fs/fat/file.c:395
 fat_write_failed fs/fat/inode.c:218 [inline]
 fat_write_end+0xba/0x160 fs/fat/inode.c:246
 generic_perform_write+0x312/0x490 mm/filemap.c:4263
 __generic_file_write_iter+0xec/0x120 mm/filemap.c:4356
 generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4385
 do_iter_readv_writev+0x4a1/0x540 fs/read_write.c:-1
 vfs_writev+0x2df/0x8b0 fs/read_write.c:1057
 do_writev+0xe7/0x210 fs/read_write.c:1103
 __do_sys_writev fs/read_write.c:1171 [inline]
 __se_sys_writev fs/read_write.c:1168 [inline]
 __x64_sys_writev+0x45/0x50 fs/read_write.c:1168
 x64_sys_call+0x1e9a/0x3000 arch/x86/include/generated/asm/syscalls_64.h:21
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811c5b1c00 of 1024 bytes by task 5586 on cpu 0:
 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:306 [inline]
 iterate_and_advance include/linux/iov_iter.h:330 [inline]
 __copy_from_iter lib/iov_iter.c:249 [inline]
 copy_folio_from_iter_atomic+0x7fc/0x11b0 lib/iov_iter.c:479
 generic_perform_write+0x2c2/0x490 mm/filemap.c:4260
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3464
 lo_rw_aio+0x6a0/0x760 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1923 [inline]
 loop_process_work+0x52d/0xa60 drivers/block/loop.c:1958
 loop_workfn+0x31/0x40 drivers/block/loop.c:1982
 process_one_work kernel/workqueue.c:3263 [inline]
 process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3346
 worker_thread+0x582/0x770 kernel/workqueue.c:3427
 kthread+0x489/0x510 kernel/kthread.c:463
 ret_from_fork+0x122/0x1b0 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: 5586 Comm: kworker/u8:35 Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
Workqueue: loop0 loop_workfn
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