syzbot

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

write to 0xffff8881592708bc of 4 bytes by task 23299 on cpu 0:
 __fat_write_inode+0x236/0x520 fs/fat/inode.c:-1
 fat_write_inode+0xbf/0xd0 fs/fat/inode.c:922
 write_inode fs/fs-writeback.c:1525 [inline]
 __writeback_single_inode+0x319/0x7c0 fs/fs-writeback.c:1745
 writeback_single_inode+0x167/0x3e0 fs/fs-writeback.c:1801
 sync_inode_metadata+0x5b/0x90 fs/fs-writeback.c:2883
 writeback_inode fs/fat/inode.c:1868 [inline]
 fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1888
 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:244
 generic_perform_write+0x312/0x490 mm/filemap.c:4124
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4220
 generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4246
 new_sync_write fs/read_write.c:591 [inline]
 vfs_write+0x4a0/0x8d0 fs/read_write.c:684
 ksys_write+0xda/0x1a0 fs/read_write.c:736
 __do_sys_write fs/read_write.c:747 [inline]
 __se_sys_write fs/read_write.c:744 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:744
 x64_sys_call+0x2cdd/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:2
 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 0xffff888159270800 of 2048 bytes by task 31 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+0x5fa/0x7c0 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1889 [inline]
 loop_process_work+0x52d/0xa60 drivers/block/loop.c:1924
 loop_workfn+0x31/0x40 drivers/block/loop.c:1948
 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: 31 Comm: kworker/u8:1 Not tainted 6.15.0-rc7-syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/19/2025
Workqueue: loop2 loop_workfn
==================================================================