From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753158AbcHACgq (ORCPT ); Sun, 31 Jul 2016 22:36:46 -0400 Received: from relay5-d.mail.gandi.net ([217.70.183.197]:51399 "EHLO relay5-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753067AbcHACgk (ORCPT ); Sun, 31 Jul 2016 22:36:40 -0400 X-Originating-IP: 89.177.44.58 To: "linux-kernel@vger.kernel.org" From: Tomas Vondra Subject: Dirty/Writeback fields in /proc/meminfo affected by 20d74bf29c Message-ID: <80b21fe4-ee8b-314c-ee3e-c09386bf368d@pgaddict.com> Date: Mon, 1 Aug 2016 04:36:28 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:49.0) Gecko/20100101 Thunderbird/49.0a2 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, While investigating a strange OOM issue on the 3.18.x branch (which turned out to be already fixed by 52c84a95), I've noticed a strange difference in Dirty/Writeback fields in /proc/meminfo depending on kernel version. I'm wondering whether this is expected ... I've bisected the change to 20d74bf29c, added in 3.18.22 (upstream commit 4f258a46): sd: Fix maximum I/O size for BLOCK_PC requests With /etc/sysctl.conf containing vm.dirty_background_bytes = 67108864 vm.dirty_bytes = 1073741824 a simple "dd" example writing 10GB file dd if=/dev/zero of=ssd.test.file bs=1M count=10240 results in about this on 3.18.21: Dirty: 740856 kB Writeback: 12400 kB but on 3.18.22: Dirty: 49244 kB Writeback: 656396 kB I.e. it seems to revert the relationship. I haven't identified any performance impact, and apparently for random writes the behavior did not change at all (or at least I haven't managed to reproduce it). But it's unclear to me why setting a maximum I/O size should affect this, and perhaps it has impact that I don't see. regards Tomas