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

[Rik van Riel]

On Wed, 30 Apr 2014 12:35:26 -0700
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
---
 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;
 

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