Messages in this thread Patch in this message | | | Date | Sun, 20 Jan 2013 11:02:10 +1100 | From | paul.szabo@sydney ... | 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
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>
--- mm/page-writeback.c.old 2012-12-06 22:20:40.000000000 +1100 +++ mm/page-writeback.c 2013-01-20 07:47:55.000000000 +1100 @@ -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;
| |