linux-kernel - Re: [patch 4/8] mm: memcg: lookup_page

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Message-ID: <20111128091703.GB9356@cmpxchg.org>
Date:	Mon, 28 Nov 2011 10:17:03 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Balbir Singh <bsingharora@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 4/8] mm: memcg: lookup_page_cgroup (almost) never returns
 NULL

On Mon, Nov 28, 2011 at 12:33:31PM +0530, Balbir Singh wrote:
> On Wed, Nov 23, 2011 at 9:12 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> > From: Johannes Weiner <jweiner@...hat.com>
> >
> > Pages have their corresponding page_cgroup descriptors set up before
> > they are used in userspace, and thus managed by a memory cgroup.
> >
> > The only time where lookup_page_cgroup() can return NULL is in the
> > page sanity checking code that executes while feeding pages into the
> > page allocator for the first time.
> >
> 
> This is a legacy check from the days when we allocated PC during fault
> time on demand. It might make sense to assert on !pc in DEBUG_VM mode
> at some point in the future

I don't think a BUG_ON bears more information than a null-pointer
dereference.
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