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

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Message-ID: <20110119121043.GB2232@cmpxchg.org>
Date:	Wed, 19 Jan 2011 13:10:43 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
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

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?

	Hannes
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