From ed84825b785ceb932af7dd5aa08614801721320b Mon Sep 17 00:00:00 2001 From: "paul.szabo@sydney.edu.au" Date: Sun, 20 Jan 2013 11:02:10 +1100 Subject: [PATCH] Negative (setpoint-dirty) in bdi_position_ratio() In bdi_position_ratio(), get difference (setpoint-dirty) right even when negative. Both setpoint and dirty are unsigned long, the difference was zero-padded thus wrongly sign-extended to s64. This issue affects all 32-bit architectures, does not affect 64-bit architectures where long and s64 are equivalent. In this function, dirty is between freerun and limit, the pseudo-float x is between [-1,1], expected to be negative about half the time. With zero-padding, instead of a small negative x we obtained a large positive one so bdi_position_ratio() returned garbage. Casting the difference to s64 also prevents overflow with left-shift; though normally these numbers are small and I never observed a 32-bit overflow there. (This patch does not solve the PAE OOM issue.) Paul Szabo psz@maths.usyd.edu.au http://www.maths.usyd.edu.au/u/psz/ School of Mathematics and Statistics University of Sydney Australia Reviewed-by: Jan Kara Reported-by: Paul Szabo Reference: http://bugs.debian.org/695182 Signed-off-by: Paul Szabo Signed-off-by: Fengguang Wu --- mm/page-writeback.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 0713bfbf0954..1534ebd6e70f 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -692,7 +692,7 @@ 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((setpoint - dirty) << RATELIMIT_CALC_SHIFT, + x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, limit - setpoint + 1); pos_ratio = x; pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; -- 2.39.5