[syzbot] [exfat?] WARNING in fat12_ent_put

[syzbot <syzbot+e64c6472a3d96a75172a@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    28d012efb432 Add linux-next specific files for 20260812
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=104eec79580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5a13c7bc480b5c3f
dashboard link: https://syzkaller.appspot.com/bug?extid=e64c6472a3d96a75172a
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/de882be6df4b/disk-28d012ef.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/412cb9fb7e2d/vmlinux-28d012ef.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9d7096c27f1b/bzImage-28d012ef.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e64c6472a3d96a75172a@syzkaller.appspotmail.com

Buffer I/O error on dev loop0, logical block 1, lost sync page write
------------[ cut here ]------------
!buffer_uptodate(bh)
WARNING: fs/buffer.c:991 at mark_buffer_dirty+0x299/0x410 fs/buffer.c:991, CPU#1: syz.0.824/10458
Modules linked in:
CPU: 1 UID: 0 PID: 10458 Comm: syz.0.824 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:mark_buffer_dirty+0x299/0x410 fs/buffer.c:991
Code: 4c 89 f7 e8 49 11 d8 ff 49 8b 3e be 40 00 00 00 5b 41 5c 41 5e 41 5f 5d e9 34 53 fb ff e8 9f a3 69 ff eb 8c e8 98 a3 69 ff 90 <0f> 0b 90 e9 a5 fd ff ff e8 8a a3 69 ff 90 0f 0b 90 e9 cf fd ff ff
RSP: 0018:ffffc900065273f8 EFLAGS: 00010287
RAX: ffffffff825e07c8 RBX: ffff888056e9e460 RCX: 0000000000080000
RDX: ffffc90003f49000 RSI: 0000000000001234 RDI: 0000000000001235
RBP: dffffc0000000001 R08: ffff888056e9e467 R09: 1ffff1100add3c8c
R10: dffffc0000000000 R11: ffffed100add3c8d R12: dffffc0000000000
R13: 1ffff92000ca4eb6 R14: ffffc900065275b0 R15: ffff888050e4ccf8
FS:  00007fe3dc5436c0(0000) GS:ffff888124dfc000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2e6edff8 CR3: 000000007c12c000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 mmb_mark_buffer_dirty+0x2c/0x1f0 fs/buffer.c:624
 fat12_ent_put+0x1b7/0x2f0 fs/fat/fatent.c:173
 fat_ent_write+0xb8/0x190 fs/fat/fatent.c:422
 fat_chain_add+0x409/0x8e0 fs/fat/misc.c:140
 fat_add_cluster fs/fat/inode.c:113 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x768/0xae0 fs/fat/inode.c:190
 __block_write_begin_int+0x6c2/0x1900 fs/buffer.c:2030
 block_write_begin fs/buffer.c:2144 [inline]
 cont_write_begin+0x71b/0xac0 fs/buffer.c:2506
 fat_write_begin+0x7a/0x150 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2434 [inline]
 cont_write_begin+0x2d6/0xac0 fs/buffer.c:2496
 fat_write_begin+0x7a/0x150 fs/fat/inode.c:230
 generic_cont_expand_simple+0x146/0x210 fs/buffer.c:2397
 fat_cont_expand+0x61/0x1f0 fs/fat/file.c:226
 fat_fallocate+0x22f/0x2f0 fs/fat/file.c:304
 vfs_fallocate+0x65a/0x7e0 fs/open.c:338
 ksys_fallocate fs/open.c:362 [inline]
 __do_sys_fallocate fs/open.c:367 [inline]
 __se_sys_fallocate fs/open.c:365 [inline]
 __x64_sys_fallocate+0xbf/0x110 fs/open.c:365
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe3db59e0d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fe3dc543028 EFLAGS: 00000246 ORIG_RAX: 000000000000011d
RAX: ffffffffffffffda RBX: 00007fe3db825fa0 RCX: 00007fe3db59e0d9
RDX: 0000000000000009 RSI: 0000000000000000 RDI: 0000000000000004
RBP: 00007fe3db635024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000010001 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fe3db826038 R14: 00007fe3db825fa0 R15: 00007fffaef17d98
 </TASK>


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