Re: linux: ext4 corruption with symlinks

[Luis Henriques <luis.henriques@linux.dev>]

On Mon 10 Jun 2024 06:03:58 PM +02, Ben Hutchings wrote;

> On Sun, 5 Nov 2023 16:12:41 +0000 Hervé Werner <dud225@hotmail.com>
> wrote:
>> Hello
>> 
>> I'm sorry for the delay.
>> 
>> > Are you able to reliably preoeduce the issue and can bisect it to
>> > the introducing commit?
>> I faced this issue on real data but I struggled to find a reliable
>> scenario to reproduce it. Here is what I just came up with:
>>   sudo mkfs -t ext4 -O fast_commit,inline_data /dev/sdb
>>   sudo mount /dev/sdb /mnt/
>>   sudo install -d -o myuser /mnt/annex
>>   cd /mnt/annex
>>   git init && git annex init
>>   for i in {1..2}; do
>>     for i in {1..10000}; do
>>       dd if=/dev/urandom of=file-${i} bs=1K count=1 2>/dev/null
>>     done
>>     git annex add -J cpus . >/dev/null && git annex sync -J cpus && git annex fsck -J cpus >/dev/null
>>     git rm * && git annex sync  && git annex dropunused all
>>   done
>> 
>> Then at some point the following error appears:
>>   EXT4-fs error (device sdb): ext4_map_blocks:577: inode #3942343: block 4: comm git-annex:w: lblock 1 mapped to illegal pblock 4 (length 1)
> [...]
>
> I can also reproduce this error message using the above script and:
>
> - Linux 6.10-rc2
> - A 2 GiB loopback devic instead of /dev/sdb
>
> I bisected this back to:
>
> commit 9725958bb75cdfa10f2ec11526fdb23e7485e8e4
> Author: Xin Yin <yinxin.x@bytedance.com>
> Date:   Thu Dec 23 11:23:37 2021 +0800
>  
>     ext4: fast commit may miss tracking unwritten range during ftruncate
>
> It is still possible to cleanly revert that commit from 6.10-rc2, and
> doing so removes the error message.

Because I recently fixed an issue in the fast commit code[1] I was hoping
that you were hitting the same bug.  I've executed the reproducer with the
fix (which hasn't been merged yet) and realised it's definitely a
different problem.

Debugged the issue a bit, it seems to be related with the fact that
ext4_fc_write_inode_data() isn't able to cope with the fact that
'ei->i_fc_lblk_len' is set to EXT_MAX_BLOCKS.

I'm CC'ing Harshad, maybe he has some idea.

[1] https://lore.kernel.org/all/20240529092030.9557-2-luis.henriques@linux.dev

Cheers,
-- 
Luís