Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754881Ab3J0Raf (ORCPT ); Sun, 27 Oct 2013 13:30:35 -0400 Received: from mail-ie0-f201.google.com ([209.85.223.201]:44304 "EHLO mail-ie0-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754792Ab3J0Rad (ORCPT ); Sun, 27 Oct 2013 13:30:33 -0400 From: Greg Thelen To: Tejun Heo , Christoph Lameter , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Johannes Weiner , Michal Hocko , Balbir Singh , KAMEZAWA Hiroyuki , handai.szj@taobao.com, Andrew Morton Cc: x86@kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Greg Thelen Subject: [PATCH v2 0/3] fix unsigned pcp adjustments Date: Sun, 27 Oct 2013 10:30:14 -0700 Message-Id: <1382895017-19067-1-git-send-email-gthelen@google.com> X-Mailer: git-send-email 1.8.4.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2807 Lines: 65 As of v3.11-9444-g3ea67d0 "memcg: add per cgroup writeback pages accounting" memcg use of __this_cpu_add(counter, -nr_pages) leads to incorrect statistic values because the negated nr_pages is not sign extended (counter is long, nr_pages is unsigned int). The memcg fix is __this_cpu_sub(counter, nr_pages). But that doesn't simply work because __this_cpu_sub(counter, nr_pages) was implemented as __this_cpu_add(counter, -nr_pages) which suffers the same problem. Example: unsigned int delta = 1 preempt_disable() this_cpu_write(long_counter, 0) this_cpu_sub(long_counter, delta) preempt_enable() Before this change long_counter on a 64 bit machine ends with value 0xffffffff, rather than 0xffffffffffffffff. This is because this_cpu_sub(pcp, delta) boils down to: long_counter = 0 + 0xffffffff v3.12-rc6 shows that only new memcg code is affected by this problem - the new mem_cgroup_move_account_page_stat() is the only place where an unsigned adjustment is used. All other callers (e.g. shrink_dcache_sb) already use a signed adjustment, so no problems before v3.12. Though I did not audit the stable kernel trees, so there could be something hiding in there. Patch 1 creates a test module for percpu operations which demonstrates the __this_cpu_sub() problems. This patch is independent can be discarded if there is no interest. Patch 2 fixes __this_cpu_sub() to work with unsigned adjustments. Patch 3 uses __this_cpu_sub() in memcg. An alternative smaller solution is for memcg to use: __this_cpu_add(counter, -(int)nr_pages) admitting that __this_cpu_add/sub() doesn't work with unsigned adjustments. But I felt like fixing the core services to prevent this in the future. Changes from V1: - more accurate patch titles, patch logs, and test module description now referring to per cpu operations rather than per cpu counters. - move small test code update from patch 2 to patch 1 (where the test is introduced). Greg Thelen (3): percpu: add test module for various percpu operations percpu: fix this_cpu_sub() subtrahend casting for unsigneds memcg: use __this_cpu_sub() to dec stats to avoid incorrect subtrahend casting arch/x86/include/asm/percpu.h | 3 +- include/linux/percpu.h | 8 +-- lib/Kconfig.debug | 9 +++ lib/Makefile | 2 + lib/percpu_test.c | 138 ++++++++++++++++++++++++++++++++++++++++++ mm/memcontrol.c | 2 +- 6 files changed, 156 insertions(+), 6 deletions(-) create mode 100644 lib/percpu_test.c -- 1.8.4.1 -- 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/