syzbot

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BUG: KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic

write to 0xffff88811ca018bc of 4 bytes by task 4499 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+0x324/0x7b0 fs/fs-writeback.c:1802
 writeback_single_inode+0x16d/0x3f0 fs/fs-writeback.c:1858
 sync_inode_metadata+0x6e/0xa0 fs/fs-writeback.c:2949
 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:395
 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:4335
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x5a6/0x9f0 fs/read_write.c:686
 ksys_write+0xdc/0x1a0 fs/read_write.c:738
 __do_sys_write fs/read_write.c:749 [inline]
 __se_sys_write fs/read_write.c:746 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:746
 x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811ca01800 of 2048 bytes by task 65 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:4332
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3490
 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:1947 [inline]
 loop_process_work+0x56c/0xac0 drivers/block/loop.c:1982
 loop_workfn+0x31/0x40 drivers/block/loop.c:2006
 process_one_work kernel/workqueue.c:3257 [inline]
 process_scheduled_works+0x4cd/0x9d0 kernel/workqueue.c:3340
 worker_thread+0x581/0x770 kernel/workqueue.c:3421
 kthread+0x488/0x510 kernel/kthread.c:463
 ret_from_fork+0x148/0x280 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 65 Comm: kworker/u8:5 Tainted: G        W           syzkaller #0 PREEMPT(voluntary) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Workqueue: loop0 loop_workfn
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netdevsim netdevsim3 eth0: set [1, 0] type 2 family 0 port 6081 - 0
netdevsim netdevsim3 eth1: set [1, 0] type 2 family 0 port 6081 - 0
netdevsim netdevsim3 eth2: set [1, 0] type 2 family 0 port 6081 - 0
netdevsim netdevsim3 eth3: set [1, 0] type 2 family 0 port 6081 - 0