mm/page-writeback.c: 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.  It also
will cause pos_ratio_polynom() to return the correct value when
(setpoint - limit) exceeds 2^32.

Also uninline pos_ratio_polynom, at Andrew's request.

Signed-off-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ef413492a149..a4317da60532 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;