On Mon, Aug 31, 2015 at 02:51:08PM +0800, Qu Wenruo wrote:
>
>
> Alexandru Moise wrote on 2015/08/31 09:32 +0300:
> >On Mon, Aug 31, 2015 at 09:44:49AM +0800, Qu Wenruo wrote:
> >>>> >From the perspective of users, qgroup's referenced or exclusive
> >>>> is negative,but user can not continue to write data! a workaround
> >>>> way is to cast u64 to s64 when doing qgroup reservation
> >>>
> >>>I am unable to reproduce this problem without his modification.
> >>>I could be wrong in reverting this, so I'm gonna CC Wang as well so
> >>>he is aware of this patch.
> >>
> >>The cast is a workaround for a quite old qgroup bug, which will
> >>cause excl/rfer overflow to minus.
> >>
> >>The remove of cast rfer/exel now is OK, as qgroup keeps maturing,
> >>especially after 4.2-rc1 rfer/exel will keep sane under most case
> >>(exception will be qgroup reassign and subvolume deletion, but will
> >>not case minus value even under than case).
> >
> >rfer/exel and reserved are all of type unsigned int, how exactly would
> >they overflow to minus?
>
> Due to qgroup bugs of course,
> In old implement, btrfs_find_all_roots() will not always find the
> correct roots.
>
> Causing quota to minus more bytes on existing qgroups.
>
> For example qg->rfer is 16K, btrfs_find_all_roots() think the qg
> previously own a 32K extent but not now, and qgroup accounting
> decides to decrease qg->rfer by 32K, now you get -16K, which is a
> super huge number if used as u64.
>
> >
> >>
> >>But I'm not a fan to remove it now.
> >>As qgroup still has a known huge bug for the qg->reserved part, we
> >>are aware of it and working on it actively.
> >
> >Can you tell me more about this known huge bug and how you can
> >reproduce it using the present implementation?
> >
>
> Check the fstest patch I submitted:
> https://patchwork.kernel.org/patch/7023301/
>
> Btrfs qgroup has qgroup reserved space leak problem, and under some
> case, it can also overflow to minus.(I don't have a minus
> reproducer, but it already happened several times in my test
> environment)
>
> That's what we are fixing now, trying to make it public before 4.3-rc1.
>
> Thanks,
> Qu
Thank you for the detailed explanation Qu, I will read more on your
changes and perhaps learn a thing or two.
All the respect,
Alex
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
