Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom

Michal Hocko
2014-04-30T13:48:31Z

On Wed 30-04-14 09:30:35, Rik van Riel wrote:
[...]
> Subject: 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>
> ---
> mm/page-writeback.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ef41349..f98a297 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 long divisor;
> long long pos_ratio;
> long x;
>
> + divisor = limit - setpoint;
> + if (!divisor)
> + divisor = 1; /* Avoid div-by-zero */
> +

This is still prone to u64 -> s32 issue, isn't it?
What was the original problem anyway? Was it really setpoint > limit or
rather the overflow?

> 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;
> @@ -842,8 +847,12 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
> x_intercept = bdi_setpoint + span;
>
> if (bdi_dirty < x_intercept - span / 4) {
> + unsigned long divisor = x_intercept - bdi_setpoint;

Same here.

> + if (!divisor)
> + divisor = 1; /* Avoid div-by-zero */
> +
> pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
> - x_intercept - bdi_setpoint + 1);
> + divisor);
> } else
> pos_ratio /= 4;
>

--
Michal Hocko
SUSE Labs