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

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

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>
Cc: stable@vger.kernel.org
---
 mm/page-writeback.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ef41349..2682516 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 int divisor;
 	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;