RE: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom

[Motohiro Kosaki <Motohiro.Kosaki@us.fujitsu.com>]

> -----Original Message-----
> From: Rik van Riel [mailto:riel@redhat.com]
> Sent: Tuesday, April 29, 2014 3:19 PM
> To: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org; sandeen@redhat.com; akpm@linux-foundation.org; jweiner@redhat.com; Motohiro Kosaki JP;
> mhocko@suse.cz; fengguang.wu@intel.com; mpatlasov@parallels.com
> Subject: [PATCH] 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.
> 
> Signed-off-by: Rik van Riel <riel@redhat.com>
> Cc: stable@vger.kernel.org

Fairly obvious fix.

Acked-by: KOSAKI Motohiro <Kosaki.motohiro@jp.fujitsu.com>


> ---
>  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;