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

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Date:	Mon, 31 Aug 2015 09:32:52 +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 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?

> 
> 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?

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