linux-ext4 - Re: generic quota vs xfs quota

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Date:   Fri, 27 Apr 2018 11:20:38 +0800
From:   Liu Bo <obuil.liubo@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: generic quota vs xfs quota

On Fri, Apr 27, 2018 at 1:22 AM, Theodore Y. Ts'o <tytso@....edu> wrote:
> On Thu, Apr 26, 2018 at 04:16:08PM +0800, Liu Bo wrote:
>>
>> and ext4 ->statfs() only displays dqb_curspace while xfs ->statfs()
>> displays both,
>
> I'm not sure what you are asking.  statfs(2) has nothing to do with
> quota support.
>
> As far as reserved delayed allocation blocks is concerned, ext4
> deducts the space from the available space in the file system and from
> the user's quota when the user writes to a file and when the space is
> reserved for delayed allocation.  My understanding is that xfs does
> the same thing.
>
> Also, the linux native quota predates XFS's quota system (which came
> from Irix), so there are some subtle differences between them,
> although we do try keep things as consistent as possible.  For
> example, only recently we added support for project quota to ext4,
> which is something that used to be XFS only.  However, there is a lot
> of complexity in terms of the low-level system interfaces for quota.
> Most of this is hidden in the quota-tools userspace package.
>
> Does this help?
>

Thanks a lot for the reply.

Yes, it's about pjquota, I should have mentioned in the question.

Regarding pjquota, what confuses me is that ext4_statfs() somehow
ignores dq_dqb.dqb_rsvspace

ext4_statfs()
#ifdef CONFIG_QUOTA
    ext4_statfs_project()
        ...
        if (limit && buf->f_blocks > limit) {
              curblock = dquot->dq_dqb.dqb_curspace >> sb->s_blocksize_bits;
              buf->f_blocks = limit;
              buf->f_bfree = buf->f_bavail =
                         (buf->f_blocks > curblock) ?
                         (buf->f_blocks - curblock) : 0;
       }
       ...
#endif



thanks,
liubo