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

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Message-ID: <53602C2B.50604@redhat.com>
Date:	Tue, 29 Apr 2014 18:48:11 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 04/29/2014 06:39 PM, Andrew Morton wrote:
> 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?

I looked at div_*64, and the second argument is a 32 bit
variable. I guess a long would be ok, since if we are
dividing by more than 4 billion we don't really care :)

static inline s64 div_s64(s64 dividend, s32 divisor)

> --- 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 */

Works for me :)

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