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Commit d5c9fde3 authored by Rik van Riel's avatar Rik van Riel Committed by Linus Torvalds
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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: default avatarRik van Riel <riel@redhat.com>
Reviewed-by: default avatarMichal 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 457c1b27

mm/page-writeback.c

+3 −3
Original line number Diff line number Diff line
@@ -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;