From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx189.postini.com [74.125.245.189]) by kanga.kvack.org (Postfix) with SMTP id 91A686B0005 for ; Sat, 19 Jan 2013 19:02:34 -0500 (EST) Date: Sun, 20 Jan 2013 11:02:10 +1100 From: paul.szabo@sydney.edu.au Message-Id: <201301200002.r0K02Atl031280@como.maths.usyd.edu.au> Subject: [PATCH] Negative (setpoint-dirty) in bdi_position_ratio() Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org Cc: 695182@bugs.debian.org, linux-kernel@vger.kernel.org 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 Reference: http://bugs.debian.org/695182 Signed-off-by: Paul Szabo --- 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org