linux-kernel - Re: [PATCH v5] mm,writeback: fix divide by zero in pos_ratio

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Date:	Thu, 8 May 2014 19:17:05 +0900
From:	Masayoshi Mizuma <m.mizuma@...fujitsu.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, <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 v5] mm,writeback: fix divide by zero in pos_ratio_polynom

Hi Rik, 

On Wed, 30 Apr 2014 16:42:55 -0400 Rik van Riel wrote:
> On Wed, 30 Apr 2014 13:13:53 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
>> This was a consequence of 64->32 truncation and it can't happen any
>> more, can it?
> 
> Andrew, this is cleaner indeed :)
> 
> Masayoshi-san, does the bug still happen with this version, or does
> this fix the problem?

I applied the v5 patch and the divide error did not happen when I ran the test.
Thank you for fixing it!

Thanks,
Masayoshi Mizuma

> 
> ---8<---
> 
> Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
> 
> It is possible for "limit - setpoint + 1" to equal zero, after
> getting truncated to a 32 bit variable, and resulting in a divide
> by zero error.
> 
> Using the fully 64 bit divide functions avoids this problem.
> 
> Also uninline pos_ratio_polynom, at Andrew's request.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>   mm/page-writeback.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ef41349..a4317da 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -593,14 +593,14 @@ 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)
>   {
>   	long long pos_ratio;
>   	long x;
>   
> -	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> +	x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
>   		    limit - setpoint + 1);
>   	pos_ratio = x;
>   	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> @@ -842,7 +842,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
>   	x_intercept = bdi_setpoint + span;
>   
>   	if (bdi_dirty < x_intercept - span / 4) {
> -		pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
> +		pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
>   				    x_intercept - bdi_setpoint + 1);
>   	} else
>   		pos_ratio /= 4;
> 
> --
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