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

[Rik van Riel <riel@redhat.com>]

On Wed, 30 Apr 2014 19:01:11 +0900
Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> wrote:

> Hi Rik,
> 
> I applied your patch to linux-next kernel, then divide error happened
> when I ran ltp stress test.
> The divide error occurred on the following div_u64(), so the following
> should be also fixed...
> 
> static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,

Good catch.  This patch fixes both places, and also has Andrew's
improvements in both places.

Andrew, this can drop in -mm instead of my previous patch and
your two cleanups to it.

---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 | 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 */
+
 	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;
+		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;
 

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