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

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Date:	Tue, 29 Apr 2014 15:39:36 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sandeen@...hat.com, jweiner@...hat.com,
	kosaki.motohiro@...fujitsu.com, mhocko@...e.cz,
	fengguang.wu@...el.com, mpatlasov@...allels.com
Subject: Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom

On Tue, 29 Apr 2014 15:19:10 -0400 Rik van Riel <riel@...hat.com> wrote:

> 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.
> 
> ...
>
> --- 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 int divisor;

I'm thinking this would be better as a ulong so I don't have to worry
my pretty head over truncation things?

>  	long long pos_ratio;
>  	long x;
>  
> +	divisor = limit - setpoint;
> +	if (!divisor)
> +		divisor = 1;
> +
>  	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;

--- a/mm/page-writeback.c~mm-page-writebackc-fix-divide-by-zero-in-pos_ratio_polynom-fix
+++ a/mm/page-writeback.c
@@ -597,13 +597,13 @@ static inline long long pos_ratio_polyno
 					  unsigned long dirty,
 					  unsigned long limit)
 {
-	unsigned int divisor;
+	unsigned long divisor;
 	long long pos_ratio;
 	long x;
 
 	divisor = limit - setpoint;
 	if (!divisor)
-		divisor = 1;
+		divisor = 1;	/* Avoid div-by-zero */
 
 	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
 		    divisor);
_

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