linux-kernel - Re: [PATCH v4] mm,writeback: fix divide by zero in pos_ratio

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Message-Id: <20140430131353.fa9f49604ea39425bc93c24a@linux-foundation.org>
Date:	Wed, 30 Apr 2014 13:13:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sandeen@...hat.com, jweiner@...hat.com,
	kosaki.motohiro@...fujitsu.com, fengguang.wu@...el.com,
	mpatlasov@...allels.com, Motohiro.Kosaki@...fujitsu.com
Subject: Re: [PATCH v4] mm,writeback: fix divide by zero in
 pos_ratio_polynom

On Wed, 30 Apr 2014 16:02:18 -0400 Rik van Riel <riel@...hat.com> wrote:

> I believe this should do the trick.
> 
> ---8<---
> 
> 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.
> 

Changelog is a bit stale.

> ---
>  mm/page-writeback.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ef41349..37f56bb 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -593,15 +593,20 @@ unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
>   * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
>   *     => fast response on large errors; small oscillation near setpoint
>   */
> -static inline long long pos_ratio_polynom(unsigned long setpoint,
> +static long long pos_ratio_polynom(unsigned long setpoint,
>  					  unsigned long dirty,
>  					  unsigned long limit)
>  {
> +	unsigned long divisor;
>  	long long pos_ratio;
>  	long x;
>  
> -	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> -		    limit - setpoint + 1);
> +	divisor = limit - setpoint;
> +	if (!divisor)
> +		divisor = 1;	/* Avoid div-by-zero */

This was a consequence of 64->32 truncation and it can't happen any
more, can it?


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