[syzbot] [ext4?] WARNING in ext4_do_writepages

[syzbot] [ext4?] WARNING in ext4_do_writepages

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    6b4b543fa187 Merge tag 'parisc-for-7.4-rc1' of git://git.k..
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=17423721580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=34fc2a03f519bd67
dashboard link: https://syzkaller.appspot.com/bug?extid=97fbc97fdd4e4fa2ed3a
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12257b0d580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10c23721580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/4286db2c74d4/disk-6b4b543f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c7bad85f1b9a/vmlinux-6b4b543f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/cb2301e1bf76/bzImage-6b4b543f.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/60f2e9127825/mount_0.gz
  fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=11a9250d580000)

AI-generated root cause summary:
In ext4 writeback (mpage_map_one_extent), a WARN_ON_ONCE(!err) is hit because
ext4_map_blocks() returns 0 (indicating a hole) for an extent that was queued
for writeback by mpage_add_bh_to_extent(). This occurs when a buffer head
remains marked mapped, dirty, and delayed/unwritten (for example, during file
truncation down via ftruncate/ext4_block_zero_eof or direct I/O fallback paths),
while the extent status tree or on-disk extent mapping for the logical block is
or becomes a hole. When ext4_map_blocks() is called with
EXT4_GET_BLOCKS_IO_SUBMIT, it avoids allocating blocks for holes and returns 0,
triggering the warning and returning -EINVAL. In production kernels, the warning
does not halt execution and error recovery safely invalidates the page without
causing memory corruption, information leaks, or denial of service.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=97fbc97fdd4e4fa2ed3a

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+97fbc97fdd4e4fa2ed3a@syzkaller.appspotmail.com

------------[ cut here ]------------
!err
WARNING: fs/ext4/inode.c:2512 at mpage_map_one_extent fs/ext4/inode.c:2512 [inline], CPU#1: kworker/u8:5/72
WARNING: fs/ext4/inode.c:2512 at mpage_map_and_submit_extent fs/ext4/inode.c:2601 [inline], CPU#1: kworker/u8:5/72
WARNING: fs/ext4/inode.c:2512 at ext4_do_writepages+0x33b6/0x44b0 fs/ext4/inode.c:3077, CPU#1: kworker/u8:5/72
Modules linked in:
CPU: 1 UID: 0 PID: 72 Comm: kworker/u8:5 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
Workqueue: writeback wb_workfn (flush-7:0)
RIP: 0010:mpage_map_one_extent fs/ext4/inode.c:2512 [inline]
RIP: 0010:mpage_map_and_submit_extent fs/ext4/inode.c:2601 [inline]
RIP: 0010:ext4_do_writepages+0x33b6/0x44b0 fs/ext4/inode.c:3077
Code: 0f 0b 90 e9 b6 d9 ff ff e8 c7 d4 38 ff 90 0f 0b 90 e9 e6 dc ff ff e8 b9 d4 38 ff 90 0f 0b 90 e9 94 df ff ff e8 ab d4 38 ff 90 <0f> 0b 90 bb ea ff ff ff e9 ae f8 ff ff e8 98 d4 38 ff 48 8d 3d 11
RSP: 0018:ffffc9000158ec60 EFLAGS: 00010293
RAX: ffffffff828f01e5 RBX: 0000000000000000 RCX: ffff88801eb7ddc0
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc9000158f070 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffed10054fc8bc R12: ffffc9000158f120
R13: dffffc0000000000 R14: 0000000000000000 R15: 1ffff110054fc8d1
FS:  0000000000000000(0000) GS:ffff888124834000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f6c27e15b20 CR3: 0000000032ea4000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 ext4_writepages+0x1fe/0x4e0 fs/ext4/inode.c:3171
 do_writepages+0x338/0x560 mm/page-writeback.c:2560
 __writeback_single_inode+0x136/0x1230 fs/fs-writeback.c:1800
 writeback_sb_inodes+0x9db/0x1aa0 fs/fs-writeback.c:2111
 __writeback_inodes_wb+0x114/0x240 fs/fs-writeback.c:2187
 wb_writeback+0x42f/0xad0 fs/fs-writeback.c:2298
 wb_check_old_data_flush fs/fs-writeback.c:2402 [inline]
 wb_do_writeback fs/fs-writeback.c:2481 [inline]
 wb_workfn+0xc98/0x10f0 fs/fs-writeback.c:2509
 process_one_work kernel/workqueue.c:3495 [inline]
 process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3578
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3659
 kthread+0x38b/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


---
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