| From | Ben Hutchings <> | Date | Sat, 07 Jun 2014 02:26:29 +0100 | Subject | [PATCH 3.2 59/92] Negative (setpoint-dirty) in bdi_position_ratio() |
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3.2.60-rc1 review patch. If anyone has any objections, please let me know.
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From: "paul.szabo@sydney.edu.au" <paul.szabo@sydney.edu.au>
commit ed84825b785ceb932af7dd5aa08614801721320b upstream.
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 <jack@suse.cz> Reported-by: Paul Szabo <psz@maths.usyd.edu.au> Reference: http://bugs.debian.org/695182 Signed-off-by: Paul Szabo <psz@maths.usyd.edu.au> Signed-off-by: Fengguang Wu <fengguang.wu@intel.com> Signed-off-by: Ben Hutchings <ben@decadent.org.uk> --- mm/page-writeback.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -559,7 +559,7 @@ static unsigned long bdi_position_ratio( * => 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;
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