Re: [PATCH] Negative (setpoint-dirty) in bdi_position_ratio()

[Jan Kara <jack@suse.cz>]

On Sun 20-01-13 11:02:10, paul.szabo@sydney.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@maths.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@maths.usyd.edu.au>
> Reference: http://bugs.debian.org/695182
> Signed-off-by: Paul Szabo <psz@maths.usyd.edu.au>
  Ah, good catch. Thanks for the patch. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>

  I've also added CC to writeback maintainer.

								Honza

> 
> --- mm/page-writeback.c.old	2012-12-06 22:20:40.000000000 +1100
> +++ mm/page-writeback.c	2013-01-20 07:47:55.000000000 +1100
> @@ -559,7 +559,7 @@ static unsigned long bdi_position_ratio(
>  	 *     => fast response on large errors; small oscillation near setpoint
>  	 */
>  	setpoint = (freerun + limit) / 2;
> -	x = div_s64((setpoint - dirty) << RATELIMIT_CALC_SHIFT,
> +	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
>  		    limit - setpoint + 1);
>  	pos_ratio = x;
>  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> --
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-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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