linux-kernel - Re: [PATCH 2/4] memcg: fix USED bit handling at uncharge in THP

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Message-Id: <20110120095503.74dea304.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 20 Jan 2011 09:55:03 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/4] memcg: fix USED bit handling at uncharge in THP

On Wed, 19 Jan 2011 13:10:43 +0100
Johannes Weiner <hannes@...xchg.org> wrote:

> Hello KAMEZAWA-san,
> 
> On Tue, Jan 18, 2011 at 11:40:49AM +0900, KAMEZAWA Hiroyuki wrote:
> > +void mem_cgroup_split_huge_fixup(struct page *head, struct page *tail)
> > +{
> > +	struct page_cgroup *head_pc = lookup_page_cgroup(head);
> > +	struct page_cgroup *tail_pc = lookup_page_cgroup(tail);
> > +	unsigned long flags;
> > +
> > +	/*
> > +	 * We have no races witch charge/uncharge but will have races with
> > +	 * page state accounting.
> > +	 */
> > +	move_lock_page_cgroup(head_pc, &flags);
> > +
> > +	tail_pc->mem_cgroup = head_pc->mem_cgroup;
> > +	smp_wmb(); /* see __commit_charge() */
> 
> I thought the barriers were needed because charging does not hold the
> lru lock.  But here we do, and all the 'lockless' read-sides do so as
> well.  Am I missing something or can this barrier be removed?
> 

Hmm. I think this can be removed. But it's just because there is only one
referer of lockless access to pc->mem_cgroup. I think it's ok to remove
this but it should be done by independent patch with enough patch
clarification. IOW, I'll do later in another patch.

Thanks,
-Kame

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