linux-kernel - Re: [PATCH] btrfs: Remove unneeded cast to s64 for qgroup rfer state

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Date:	Mon, 31 Aug 2015 09:56:37 +0300
From:	Alexandru Moise <00moses.alexander00@...il.com>
To:	Qu Wenruo <quwenruo@...fujitsu.com>
Cc:	clm@...com, jbacik@...com, dsterba@...e.com,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: Remove unneeded cast to s64 for qgroup rfer state

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
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