Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2993536AbbEPIt3 (ORCPT ); Sat, 16 May 2015 04:49:29 -0400 Received: from bband-dyn183.178-41-215.t-com.sk ([178.41.215.183]:22401 "EHLO ip4-83-240-18-248.cust.nbox.cz" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1946331AbbEPIDr (ORCPT ); Sat, 16 May 2015 04:03:47 -0400 From: Jiri Slaby To: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Tejun Heo , Jens Axboe , Jiri Slaby Subject: [PATCH 3.12 119/142] writeback: use |1 instead of +1 to protect against div by zero Date: Sat, 16 May 2015 09:37:59 +0200 Message-Id: <62d7a1a87954f16f6c43d345f347afc2a0ee80ac.1431761807.git.jslaby@suse.cz> X-Mailer: git-send-email 2.3.7 In-Reply-To: <70c3d4ae1322b9e9bd7443ef574af5635234a0fa.1431761807.git.jslaby@suse.cz> References: <70c3d4ae1322b9e9bd7443ef574af5635234a0fa.1431761807.git.jslaby@suse.cz> In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2806 Lines: 75 From: Tejun Heo 3.12-stable review patch. If anyone has any objections, please let me know. =============== commit 464d1387acb94dc43ba772b35242345e3d2ead1b upstream. mm/page-writeback.c has several places where 1 is added to the divisor to prevent division by zero exceptions; however, if the original divisor is equivalent to -1, adding 1 leads to division by zero. There are three places where +1 is used for this purpose - one in pos_ratio_polynom() and two in bdi_position_ratio(). The second one in bdi_position_ratio() actually triggered div-by-zero oops on a machine running a 3.10 kernel. The divisor is x_intercept - bdi_setpoint + 1 == span + 1 span is confirmed to be (u32)-1. It isn't clear how it ended up that but it could be from write bandwidth calculation underflow fixed by c72efb658f7c ("writeback: fix possible underflow in write bandwidth calculation"). At any rate, +1 isn't a proper protection against div-by-zero. This patch converts all +1 protections to |1. Note that bdi_update_dirty_ratelimit() was already using |1 before this patch. Signed-off-by: Tejun Heo Reviewed-by: Jan Kara Signed-off-by: Jens Axboe Signed-off-by: Jiri Slaby --- mm/page-writeback.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 51d8d15f48d7..656a5490f693 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -601,7 +601,7 @@ static long long pos_ratio_polynom(unsigned long setpoint, long x; x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, - limit - setpoint + 1); + (limit - setpoint) | 1); pos_ratio = x; pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; @@ -828,7 +828,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi, * scale global setpoint to bdi's: * bdi_setpoint = setpoint * bdi_thresh / thresh */ - x = div_u64((u64)bdi_thresh << 16, thresh + 1); + x = div_u64((u64)bdi_thresh << 16, thresh | 1); bdi_setpoint = setpoint * (u64)x >> 16; /* * Use span=(8*write_bw) in single bdi case as indicated by @@ -843,7 +843,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi, if (bdi_dirty < x_intercept - span / 4) { pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty), - x_intercept - bdi_setpoint + 1); + (x_intercept - bdi_setpoint) | 1); } else pos_ratio /= 4; -- 2.3.7 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/