mm/page-writeback.c: fix divide by zero in pos_ratio_polynom

commit d5c9fde3dae750889168807038243ff36431d276 upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2:
 Adjust context - pos_ratio_polynom() is not a separate function]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 022622dbf42df46349febc1f6a466a38a60a5a84..d2ac057008f2a9e3f4859ec07f31cf82d5173d68 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -559,7 +559,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
         *     => fast response on large errors; small oscillation near setpoint
         */
        setpoint = (freerun + limit) / 2;
-       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;
@@ -625,7 +625,7 @@ 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;