linux-ext4 - Re: Journaled quota

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Date:   Tue, 28 Aug 2018 12:00:58 -0700
From:   Liu Bo <obuil.liubo@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: Journaled quota

On Tue, Aug 28, 2018 at 1:27 AM, Jan Kara <jack@...e.cz> wrote:
> Hi!
>
> On Mon 27-08-18 12:01:10, Liu Bo wrote:
>> I've run into a problem that "repquota" doesn't always show the latest
>> quota stats when using journaled quota of v2, "aquota.group".
>>
>> Here's the steps:
>> 1. quotacheck -cug /mnt
>> 2. quotaon -g /mnt
>> 3. setquota -g 0 0 2G 0 0 /mnt
>> 4. repquota -g /mnt
>>
>> # in step 4, it didn't always show 2G as the hardlimit
>>
>> I straced repquota and had realized that it does call both ->sync_fs()
>> and sync_blockdev() and then truncates quota file's page cache to
>> avoid userspace programs reading stale quota stats.  However, the
>> changes that "setquota" writes to quota files are stored as metadata
>> in ext4 and the "bh" is then marked as "jbddirty".  At the ->sync_fs()
>> time, if the bh is modified by a later transaction while committing
>> ext4's journal, the journal bh "jh" makes it to disk while the "bh" is
>> not, and the "bh" is again queued in BJ_Metadata list and marked
>> "jbddirty", the point is "bh" is not marked as dirty so that
>> sync_blockdev() is not able to flush it to disk, then the end result
>> is that quota file "aquota.group" still has stale data which
>> "repquota" is reading and showing.
>>
>> I found the above in an old 3.x kernel, but I've checked that the
>> logic also applied to 4.18.
>>
>> I was wondering if it was a known problem or am I missing anything?
>
> I was not aware of this problem but you are right that if the buffer with
> latest quota data gets modified after repquota(8) calls ->sync_fs() and
> before it calls sync_blockdev(), then sync_blockdev() won't checkpoint the
> data (write them to final location on disk) and thus repquota(8) won't see
> them when reading quota information. This is inherent to how JBD2
> journalling works and is difficult to avoid (the only way I currently see
> is to freeze the journal which also forces checkpointing of all the
> transactions but that is much more heavyweight operation).
>
> The problem with data visibility in aquota.* files also exists in 4.18
> kernel, however newer kernels (and quota tools) have different way of
> reporting quotas - they have Q_GETNEXTQUOTA quotactl which repquota(8) will
> use if available and that fixes this problem as repquota(8) no longer needs
> to read quota files directly.
>

I see, so Q_GETNEXTQUOTA will call do_get_dqblk() to get the in-memory
latest quota.

Will check the latest tool, thanks for the reply.

thanks,
liubo