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

Maxim Patlasov
2014-04-30T08:04:20Z

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@redhat.com>
> Cc: stable@vger.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;