From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f44.google.com (mail-wg0-f44.google.com [74.125.82.44]) by kanga.kvack.org (Postfix) with ESMTP id DC7F890008B for ; Thu, 30 Oct 2014 10:14:18 -0400 (EDT) Received: by mail-wg0-f44.google.com with SMTP id x12so3816460wgg.31 for ; Thu, 30 Oct 2014 07:14:18 -0700 (PDT) Received: from gum.cmpxchg.org (gum.cmpxchg.org. [85.214.110.215]) by mx.google.com with ESMTPS id he2si10217759wjc.174.2014.10.30.07.14.17 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 30 Oct 2014 07:14:17 -0700 (PDT) Date: Thu, 30 Oct 2014 10:14:01 -0400 From: Johannes Weiner Subject: Re: [PATCH] mm: initialize variable for mem_cgroup_end_page_stat Message-ID: <20141030141401.GA24520@phnom.home.cmpxchg.org> References: <1414633464-19419-1-git-send-email-sasha.levin@oracle.com> <20141030082712.GB4664@dhcp22.suse.cz> <54523DDE.9000904@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54523DDE.9000904@oracle.com> Sender: owner-linux-mm@kvack.org List-ID: To: Sasha Levin Cc: Michal Hocko , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, riel@redhat.com, peterz@infradead.org, linux-mm@kvack.org On Thu, Oct 30, 2014 at 09:32:14AM -0400, Sasha Levin wrote: > On 10/30/2014 04:27 AM, Michal Hocko wrote: > > On Wed 29-10-14 21:44:24, Sasha Levin wrote: > >> > Commit "mm: memcontrol: fix missed end-writeback page accounting" has changed > >> > the behaviour of mem_cgroup_begin_page_stat() to not always set the "locked" > >> > parameter. > >> > > >> > We should initialize it at the callers to prevent garbage being used in a > >> > later call to mem_cgroup_end_page_stat(). > > The contract is that if the returned memcg is non-NULL then the locked > > is always initialized. Nobody but mem_cgroup_end_page_stat should touch > > this variable and this function makes sure it uses it properly. Similar > > applies to flags which is initialized only if we really take the slow > > path (has a meaning only if locked == true). > > > > So this is not really needed. Was this triggered by a compiler warning? > > The problem is that you are attempting to read 'locked' when you call > mem_cgroup_end_page_stat(), so it gets used even before you enter the > function - and using uninitialized variables is undefined. We are not using that value anywhere if !memcg. What path are you referring to? > Yes, it's a compiler warning. Could you provide that please, including arch, and gcc version? Thanks -- 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