linux-kernel - Re: [PATCH] Negative (setpoint-dirty) in bdi_position

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Date:	Thu, 24 Jan 2013 22:14:41 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	paul.szabo@...ney.edu.au, linux-mm@...ck.org,
	695182@...s.debian.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Negative (setpoint-dirty) in bdi_position_ratio()

On Wed, Jan 23, 2013 at 12:54:38AM +0100, Jan Kara wrote:
> On Sun 20-01-13 11:02:10, paul.szabo@...ney.edu.au wrote:
> > In bdi_position_ratio(), get difference (setpoint-dirty) right even when
> > negative. Both setpoint and dirty are unsigned long, the difference was
> > zero-padded thus wrongly sign-extended to s64. This issue affects all
> > 32-bit architectures, does not affect 64-bit architectures where long
> > and s64 are equivalent.
> > 
> > In this function, dirty is between freerun and limit, the pseudo-float x
> > is between [-1,1], expected to be negative about half the time. With
> > zero-padding, instead of a small negative x we obtained a large positive
> > one so bdi_position_ratio() returned garbage.
> > 
> > Casting the difference to s64 also prevents overflow with left-shift;
> > though normally these numbers are small and I never observed a 32-bit
> > overflow there.
> > 
> > (This patch does not solve the PAE OOM issue.)
> > 
> > Paul Szabo   psz@...hs.usyd.edu.au   http://www.maths.usyd.edu.au/u/psz/
> > School of Mathematics and Statistics   University of Sydney    Australia
> > 
> > Reported-by: Paul Szabo <psz@...hs.usyd.edu.au>
> > Reference: http://bugs.debian.org/695182
> > Signed-off-by: Paul Szabo <psz@...hs.usyd.edu.au>
>   Ah, good catch. Thanks for the patch. You can add:
> Reviewed-by: Jan Kara <jack@...e.cz>
> 
>   I've also added CC to writeback maintainer.

Applied. Thanks! It's a good fix.

Thanks,
Fengguang
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