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

[Qu Wenruo]

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
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