[PATCH v5] mm,writeback: fix divide by zero in pos_ratio_polynom

Rik van Riel
2014-04-30T20:45:37Z

On Wed, 30 Apr 2014 13:13:53 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

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

Andrew, this is cleaner indeed :)

Masayoshi-san, does the bug still happen with this version, or does
this fix the problem?

---8<---

Subject: mm,writeback: fix divide by zero in pos_ratio_polynom

It is possible for "limit - setpoint + 1" to equal zero, after
getting truncated to a 32 bit variable, and resulting in a divide
by zero error.

Using the fully 64 bit divide functions avoids this problem.

Also uninline pos_ratio_polynom, at Andrew's request.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
mm/page-writeback.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ef41349..a4317da 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -593,14 +593,14 @@ 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)
{
long long pos_ratio;
long x;

- x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
+ x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
limit - setpoint + 1);
pos_ratio = x;
pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
@@ -842,7 +842,7 @@ 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),
+ pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
x_intercept - bdi_setpoint + 1);
} else
pos_ratio /= 4;