Re: [syzbot ci] Re: ext4: use iomap for regular file's buffered I/O path

[Zhang Yi]

On 10/21/2026 4:03 PM, Ojaswin Mujoo wrote:
> On Sat, Oct 10, 2026 at 04:44:52PM +0800, Zhang Yi wrote:
>> On 10/10/2026 1:54 AM, syzbot ci wrote:
>>> syzbot ci has tested the following series
>>>
>>
>> [...]
>>
>>> and found the following issue:
>>> WARNING in ext4_do_writepages
>>>
>>> Full report is available here:
>>> https://ci.syzbot.org/series/3711c951-acf4-4ebe-a920-9cc89b6be2ff
>>>
>>> ***
>>>
>>> WARNING in ext4_do_writepages
>>>
>>> tree:      vfs
>>> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/vfs/vfs.git
>>> base:      3d399224425573875b6f6f1181bd8cf9a28cc7d1
>>> arch:      amd64
>>> compiler:  Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
>>> config:    https://ci.syzbot.org/builds/c3c01f8c-846d-4c7a-955a-fd7427af6b0d/config
>>>
>>> ------------[ cut here ]------------
>>> !err
>>> WARNING: fs/ext4/inode.c:2512 at ext4_do_writepages+0x33b3/0x44a0, CPU#0: kworker/u9:3/147
>>> Modules linked in:
>>> CPU: 0 UID: 0 PID: 147 Comm: kworker/u9:3 Not tainted syzkaller #0 PREEMPT(full) 
>>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>>> Workqueue: writeback wb_workfn (flush-7:1)
>>> RIP: 0010:ext4_do_writepages+0x33b3/0x44a0
>>> Code: 0f 0b 90 e9 b6 d9 ff ff e8 6a e2 36 ff 90 0f 0b 90 e9 e6 dc ff ff e8 5c e2 36 ff 90 0f 0b 90 e9 94 df ff ff e8 4e e2 36 ff 90 <0f> 0b 90 bb ea ff ff ff e9 ae f8 ff ff e8 3b e2 36 ff 48 8d 3d 64
>>> RSP: 0018:ffffc90003106ce0 EFLAGS: 00010293
>>> RAX: ffffffff8290e912 RBX: 0000000000000000 RCX: ffff8881053f9e00
>>> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
>>> RBP: ffffc900031070f0 R08: ffff888025da4a37 R09: 1ffff11004bb4946
>>> R10: dffffc0000000000 R11: ffffed1004bb4947 R12: ffffc900031071a0
>>> R13: dffffc0000000000 R14: 0000000000000000 R15: 1ffff11004bb495d
>>> FS:  0000000000000000(0000) GS:ffff88818d6c9000(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 00005555565a5a28 CR3: 0000000031f66000 CR4: 00000000000006f0
>>> Call Trace:
>>>  <TASK>
>>>  ext4_writepages+0x241/0x3b0
>>>  do_writepages+0x338/0x560
>>>  __writeback_single_inode+0x136/0x1220
>>>  writeback_sb_inodes+0x9de/0x1b00
>>>  wb_writeback+0x41c/0xad0
>>>  wb_workfn+0x431/0x10f0
>>>  process_scheduled_works+0xc3d/0x1630
>>>  worker_thread+0xa47/0xfb0
>>>  kthread+0x38b/0x480
>>>  ret_from_fork+0x514/0xb70
>>>  ret_from_fork_asm+0x1a/0x30
>>>  </TASK>
>>>
>>>
>>
>> This WARNING was newly introduced by patch 8, "ext4: skip block
>> allocation for holes in the data submission path". It doesn't actually
>> cause a new regression, it just appears to expose a pre-existing issue
>> in the buffer_head buffered I/O path, where an inconsistency between
>> the bh and extent state could potentially lead to allocating a block and
>> writing data into a hole. The logic of the patch itself is fine. To
>> avoid noise, I can drop the WARN_ON_ONCE() while still keeping return
>> -EINVAL. Thoughts?
> 
> Hi Zhang,
> 
> I think we can keep the WARN_ON. Failing in writeback here will anyways
> throw out some error messages about data loss so having an extra WARN
> dump should be okay. It will help us find and eliminate paths that cause
> this quickly, like your patch for the inline truncate fix.
> 

Yes, I agree. After all, now that we're encountering holes not being
written, together with the existing logging, this WARN_ON can help us catch
the problem.

Thanks,
Yi.

> Regards,
> ojaswin
> 
>>
>> Thanks,
>> Yi.
>>
>>> ***
>>>
>>> If these findings have caused you to resend the series or submit a
>>> separate fix, please add the following tag to your commit message:
>>>   Tested-by: syzbot@xxxxxxxxxxxxxxxxxxxxxxxxx
>>>
>>> ---
>>> This report is generated by a bot. It may contain errors.
>>> syzbot ci engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx.
>>>
>>> To test a fix for this bug, please reply with `#syz test`
>>> (on a separate line) and attach the patch to the email.
>>>
>>> Notes:
>>> - The patch will be applied on top of the tested series (as an
>>>   incremental fix).
>>> - To test a new version of the whole series, please send it directly
>>>   to syzbot@xxxxxxxxxxxxxxx.
>>> - Arguments like custom git repos and branches are not supported.
>>