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

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Date:	Wed, 30 Apr 2014 12:04:04 +0400
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Rik van Riel <riel@...hat.com>, <linux-kernel@...r.kernel.org>
CC:	<linux-mm@...ck.org>, <sandeen@...hat.com>,
	<akpm@...ux-foundation.org>, <jweiner@...hat.com>,
	<kosaki.motohiro@...fujitsu.com>, <mhocko@...e.cz>,
	<fengguang.wu@...el.com>
Subject: Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom

Hi Rik!

On 04/29/2014 11:19 PM, Rik van Riel 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.

The patch looks correct, but I'm afraid it can hide an actual bug in a 
caller of pos_ratio_polynom(). The latter is not intended for setpoint > 
limit. All callers take pains to ensure that setpoint <= limit. Look, 
for example, at global_dirty_limits():

 >     if (background >= dirty)
 >        background = dirty / 2;

If you ever encountered "limit - setpoint + 1" equal zero, it may be 
worthy to investigate how you came to setpoint greater than limit.

Thanks,
Maxim

>
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Cc: stable@...r.kernel.org
> ---
>   mm/page-writeback.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ef41349..2682516 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 int divisor;
>   	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;

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