mm/page-writeback.c: fix divide by zero in pos_ratio_polynom (d5c9fde3) · Commits · 戴 / test

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>

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;

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