Re: [PATCH v4] mm,writeback: fix divide by zero in pos_ratio_polynom

Andrew Morton
2014-04-30T20:13:56Z

On Wed, 30 Apr 2014 16:02:18 -0400 Rik van Riel <riel@redhat.com> wrote:

> I believe this should do the trick.
>
> ---8<---
>
> 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.
>

Changelog is a bit stale.

> ---
> mm/page-writeback.c | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ef41349..37f56bb 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -593,15 +593,20 @@ 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)
> {
> + unsigned long divisor;
> long long pos_ratio;
> long x;
>
> - x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> - limit - setpoint + 1);
> + divisor = limit - setpoint;
> + if (!divisor)
> + divisor = 1; /* Avoid div-by-zero */

This was a consequence of 64->32 truncation and it can't happen any
more, can it?