Re: Exclusive quota of snapshot exceeded despite no space used

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

>>> Thanks a lot for your reply!
>>>
>>> While remounting the filesystem fixes the issue temporary, it doesn't
>>> take very long for the bug to happen again so it's not really a
>>> workaround I can work with.
>>>
>>> I did recompile the kernel using your patches, but unfortunately the
>>> problem still appears.
>>>
>>> Thanks,
>>> Johannes
>>>
>> Interesting, just touching file will cause EQUOTA is quite a big problem.
>>
>> I'll try to reproduce it with my patchset and see what really caused
>> the problem.
>> The problem seems to do with snapshot qgroup hacking.
>> But I'm not completely sure yet.
>>
>> BTW, does "sync; btrfs qgroup show -prce" still show excl as 16K?
>> 16K is the correct number with only 6 empty files, just in case.
>>
>> Thanks,
>> Qu
>
> I ran my example from the first mail again and managed to write 7 files
> this time, "qgroup show" still shows 16kB after sync:
>
> root@t420:/media/extern/snap# btrfs qg limit -e 50M .
> root@t420:/media/extern/snap# for file in {1..100}; do touch $file;
> sleep 5m; done
> touch: cannot touch ‘8’: Disk quota exceeded
> ^C
> root@t420:/media/extern/snap# sync
> root@t420:/media/extern/snap# btrfs qgroup show -pcre .
> qgroupid         rfer         excl     max_rfer     max_excl parent  child
> --------         ----         ----     --------     -------- ------  -----
> 0/5          16.00KiB     16.00KiB         none         none ---     ---
> 0/257        16.00KiB     16.00KiB         none         none ---     ---
> 0/258        16.00KiB     16.00KiB         none     50.00MiB ---     ---
> root@t420:/media/extern/snap# btrfs fi sync .
> FSSync '.'
> root@t420:/media/extern/snap# btrfs qgroup show -pcre .
> qgroupid         rfer         excl     max_rfer     max_excl parent  child
> --------         ----         ----     --------     -------- ------  -----
> 0/5          16.00KiB     16.00KiB         none         none ---     ---
> 0/257        16.00KiB     16.00KiB         none         none ---     ---
> 0/258        16.00KiB     16.00KiB         none     50.00MiB ---     ---
>
> By the way, I don't if its relevant but the problem is not limited to
> exclusive quotas, but also happens when setting a "referenced" limit
> (qgroup limit without "-e").
>
> Thanks,
> Johannes
>

The bug is located, and turns out to be quite a stupid problem caused by 
myself.

I just forgot to include a cleanup patch during rebase!!!! AGAIN!!!

You can apply the following patch to resolve it:
[PATCH 3/3] btrfs: qgroup: Fix a rebase bug which will cause qgroup 
double free

Or just apply the whole patchset:
[4.4][PATCH 0/3] btrfs: Qgroup hotfix

At least, with the patchset based on Chris' integration-4.4 branch, it 
succeeded in touching all the 100 files in my test box.

Thanks,
Qu


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