linux-kernel - Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio

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Date:	Wed, 30 Apr 2014 15:48:26 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Rik van Riel <riel@...hat.com>
Cc:	Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sandeen@...hat.com, akpm@...ux-foundation.org, jweiner@...hat.com,
	kosaki.motohiro@...fujitsu.com, fengguang.wu@...el.com,
	mpatlasov@...allels.com, Motohiro.Kosaki@...fujitsu.com
Subject: Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom

On Wed 30-04-14 09:30:35, Rik van Riel wrote:
[...]
> Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
> 
> It is possible for "limit - setpoint + 1" to equal zero, leading to a
> divide by zero error. Blindly adding 1 to "limit - setpoint" is not
> working, so we need to actually test the divisor before calling div64.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  mm/page-writeback.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ef41349..f98a297 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -597,11 +597,16 @@ static inline long long pos_ratio_polynom(unsigned long setpoint,
>  					  unsigned long dirty,
>  					  unsigned long limit)
>  {
> +	unsigned long divisor;
>  	long long pos_ratio;
>  	long x;
>  
> +	divisor = limit - setpoint;
> +	if (!divisor)
> +		divisor = 1;	/* Avoid div-by-zero */
> +

This is still prone to u64 -> s32 issue, isn't it?
What was the original problem anyway? Was it really setpoint > limit or
rather the overflow?

>  	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> -		    limit - setpoint + 1);
> +		    divisor);
>  	pos_ratio = x;
>  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
>  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> @@ -842,8 +847,12 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
>  	x_intercept = bdi_setpoint + span;
>  
>  	if (bdi_dirty < x_intercept - span / 4) {
> +		unsigned long divisor = x_intercept - bdi_setpoint;

Same here.

> +		if (!divisor)
> +			divisor = 1;	/* Avoid div-by-zero */
> +
>  		pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
> -				    x_intercept - bdi_setpoint + 1);
> +				    divisor);
>  	} else
>  		pos_ratio /= 4;
>  

-- 
Michal Hocko
SUSE Labs
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