==================================================================
BUG: KCSAN: data-race in copy_folio_from_iter_atomic / fat16_ent_put

write to 0xffff888185962cb6 of 2 bytes by task 6427 on cpu 1:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
 fat_alloc_clusters+0x4d1/0xa80 fs/fat/fatent.c:503
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5ff/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2522 [inline]
 cont_write_begin+0x1b0/0x970 fs/buffer.c:2584
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2485
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
 fat_setattr+0x2a5/0x8a0 fs/fat/file.c:513
 notify_change+0x872/0x8f0 fs/attr.c:546
 do_truncate fs/open.c:68 [inline]
 do_ftruncate+0x335/0x3c0 fs/open.c:195
 do_sys_ftruncate fs/open.c:207 [inline]
 __do_sys_ftruncate fs/open.c:212 [inline]
 __se_sys_ftruncate fs/open.c:210 [inline]
 __x64_sys_ftruncate+0x68/0xc0 fs/open.c:210
 x64_sys_call+0x2e35/0x3000 arch/x86/include/generated/asm/syscalls_64.h:78
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888185962c00 of 512 bytes by task 37 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+0x760/0x1150 lib/iov_iter.c:491
 generic_perform_write+0x2c2/0x490 mm/filemap.c:4332
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3490
 lo_rw_aio+0x673/0x720 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1926 [inline]
 loop_process_work+0x56d/0xaa0 drivers/block/loop.c:1961
 loop_workfn+0x31/0x40 drivers/block/loop.c:1985
 process_one_work kernel/workqueue.c:3257 [inline]
 process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3340
 worker_thread+0x582/0x770 kernel/workqueue.c:3421
 kthread+0x489/0x510 kernel/kthread.c:463
 ret_from_fork+0x149/0x290 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: 37 Comm: kworker/u8:2 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: loop9 loop_workfn
==================================================================
EXT4-fs error (device loop0): ext4_mb_generate_buddy:1306: group 0, block bitmap and bg descriptor inconsistent: 25 vs 150994969 free clusters
EXT4-fs (loop0): Delayed block allocation failed for inode 15 at logical offset 0 with max blocks 160 with error 28
EXT4-fs (loop0): This should not happen!! Data will be lost

EXT4-fs (loop0): Total free blocks count 0
EXT4-fs (loop0): Free/Dirty block details
EXT4-fs (loop0): free_blocks=2415919104
EXT4-fs (loop0): dirty_blocks=160
EXT4-fs (loop0): Block reservation details
EXT4-fs (loop0): i_reserved_data_blocks=10