syzbot

==================================================================
BUG: KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic

write to 0xffff88814c941c1c of 4 bytes by task 14219 on cpu 1:
 __fat_write_inode+0x235/0x540 fs/fat/inode.c:-1
 fat_write_inode+0xbb/0xd0 fs/fat/inode.c:928
 write_inode fs/fs-writeback.c:1607 [inline]
 __writeback_single_inode+0x327/0x660 fs/fs-writeback.c:1850
 writeback_single_inode+0x16d/0x400 fs/fs-writeback.c:1906
 sync_inode_metadata+0x6e/0xa0 fs/fs-writeback.c:3080
 writeback_inode fs/fat/inode.c:1878 [inline]
 fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1898
 fat_truncate_blocks+0x536/0x5b0 fs/fat/file.c:399
 fat_write_failed fs/fat/inode.c:219 [inline]
 fat_write_end+0xba/0x160 fs/fat/inode.c:247
 generic_perform_write+0x314/0x490 mm/filemap.c:4384
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4480
 generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4506
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x57d/0x990 fs/read_write.c:687
 ksys_write+0xdc/0x1a0 fs/read_write.c:739
 __do_sys_write fs/read_write.c:750 [inline]
 __se_sys_write fs/read_write.c:747 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:747
 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+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88814c941c00 of 1024 bytes by task 8387 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+0x728/0x10a0 lib/iov_iter.c:491
 generic_perform_write+0x2c4/0x490 mm/filemap.c:4381
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3414
 lo_rw_aio+0x6a9/0x760 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1921 [inline]
 loop_process_work+0x567/0xac0 drivers/block/loop.c:1956
 loop_workfn+0x31/0x40 drivers/block/loop.c:1980
 process_one_work kernel/workqueue.c:3322 [inline]
 process_scheduled_works+0x4d4/0x9a0 kernel/workqueue.c:3405
 worker_thread+0x569/0x750 kernel/workqueue.c:3486
 kthread+0x221/0x270 kernel/kthread.c:436
 ret_from_fork+0x146/0x330 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: 8387 Comm: kworker/u8:11 Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Workqueue: loop2 loop_workfn
==================================================================