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

Rik van Riel
2014-04-30T20:05:31Z

On Wed, 30 Apr 2014 12:35:26 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> > The easy way would be by calling div64_s64 and div64_u64,
> > which are 64 bit all the way through.
> >
> > Any objections?
>
> Sounds good to me.
>
> > The inlined bits seem to be stubs calling the _rem variants
> > of the functions, and discarding the remainder.
>
> I was referring to pos_ratio_polynom(). The compiler will probably be
> uninlining it anyway, but still...

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.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
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 */
+
+ x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
+ 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) {
- pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
- x_intercept - bdi_setpoint + 1);
+ unsigned long divisor = x_intercept - bdi_setpoint;
+ if (!divisor)
+ divisor = 1; /* Avoid div-by-zero */
+
+ pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
+ divisor);
} else
pos_ratio /= 4;