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

Andrew Morton
2014-04-29T22:39:40Z

On Tue, 29 Apr 2014 15:19:10 -0400 Rik van Riel <riel@redhat.com> 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.
>
> ...
>
> --- 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;

I'm thinking this would be better as a ulong so I don't have to worry
my pretty head over truncation things?

> 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;

--- a/mm/page-writeback.c~mm-page-writebackc-fix-divide-by-zero-in-pos_ratio_polynom-fix
+++ a/mm/page-writeback.c
@@ -597,13 +597,13 @@ static inline long long pos_ratio_polyno
unsigned long dirty,
unsigned long limit)
{
- unsigned int divisor;
+ unsigned long divisor;
long long pos_ratio;
long x;

divisor = limit - setpoint;
if (!divisor)
- divisor = 1;
+ divisor = 1; /* Avoid div-by-zero */

x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
divisor);
_