On Sun 20-01-13 11:02:10, paul.szabo@xxxxxxxxxxxxx wrote: > 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@xxxxxxxxxxxxxxxxx http://www.maths.usyd.edu.au/u/psz/ > School of Mathematics and Statistics University of Sydney Australia > > Reported-by: Paul Szabo <psz@xxxxxxxxxxxxxxxxx> > Reference: http://bugs.debian.org/695182 > Signed-off-by: Paul Szabo <psz@xxxxxxxxxxxxxxxxx> Ah, good catch. Thanks for the patch. You can add: Reviewed-by: Jan Kara <jack@xxxxxxx> I've also added CC to writeback maintainer. Honza > > --- 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; > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>