linux-kernel - Re: [PATCH] mm: initialize variable for mem_cgroup_end_page

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Message-ID: <20141030141401.GA24520@phnom.home.cmpxchg.org>
Date:	Thu, 30 Oct 2014 10:14:01 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Michal Hocko <mhocko@...e.cz>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	peterz@...radead.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: initialize variable for mem_cgroup_end_page_stat

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
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