Re: Enabling quota may not correctly rescan on 4.17

[Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>]

On 2018/06/27 16:40, Nikolay Borisov wrote:
> 
> 
> On 26.06.2018 09:00, Misono Tomohiro wrote:
>> Hello Nikolay,
>>
>> I noticed that commit 5d23515be669 ("btrfs: Move qgroup rescan
>> on quota enable to btrfs_quota_enable") in 4.17 sometimes causes
>> to fail correctly rescanning quota when quota is enabled.
>>
>> Simple reproducer:
>>
>> $ mkfs.btrfs -f $DEV
>> $ mount $DEV /mnt
>> $ dd if=/dev/urandom of=/mnt/file bs=1000 count=1000
>> $ btrfs quota enbale /mnt
>> $ umount /mnt
>> $ btrfs check $DEV
>> ...
>> checking quota groups
>> Counts for qgroup id: 0/5 are different
>> our:            referenced 1019904 referenced compressed 1019904
>> disk:           referenced 16384 referenced compressed 16384
>> diff:           referenced 1003520 referenced compressed 1003520
>> our:            exclusive 1019904 exclusive compressed 1019904
>> disk:           exclusive 16384 exclusive compressed 16384
>> diff:           exclusive 1003520 exclusive compressed 1003520
>> found 1413120 bytes used, error(s) found
>> ...
> 
> I ran your script 100 times with progs 4.17 and 4.18-rc1 and didn't
> observe this error. I didn't observe btrfs/114 also failing but I ran it
> a lot less. Is there anything else i can do to make your small
> reproducer more likely to trigger?

How about btrfs/114? I saw the problem in it first (progs 4.17/kernel 4.18-rc2)
and it seems always happen in my environment. 

> 
>>
>> This can be also observed in btrfs/114. \v(Note that progs < 4.17
>> returns error code 0 even if quota is not consistency and therefore
>> test will incorrectly pass.)
>>
>> My observation is that this commit changed to call initial quota rescan
>> when quota is enabeld instead of first comit transaction after enabling
>> quota, and therefore if there is something not commited at that time,
>> their usage will not be accounted.
>>
>> Actually this can be simply fixed by calling "btrfs rescan" again or
>> calling "btrfs fi sync" before "btrfs quota enable".
>>
>> I think the commit itself makes the code much easier to read, so it may
>> be better to fix the problem in progs (i.e. calling sync before quota enable).
>>
>> Do you have any thoughts?
>>
>> Thanks,
>> Tomohiro Misono
>>
>>
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