linux-kernel - Re: [PATCH v4 06/11] memcg: add dirty page accounting infrastructure

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Date:	Fri, 29 Oct 2010 20:17:26 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Greg Thelen <gthelen@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"containers@...ts.osdl.org" <containers@...ts.osdl.org>,
	Andrea Righi <arighi@...eler.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Minchan Kim <minchan.kim@...il.com>,
	Ciju Rajan K <ciju@...ux.vnet.ibm.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v4 06/11] memcg: add dirty page accounting
 infrastructure

On Fri, 29 Oct 2010 19:13:00 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:

> On Fri, Oct 29, 2010 at 03:09:09PM +0800, Greg Thelen wrote:
> 
> > +
> > +	case MEMCG_NR_FILE_DIRTY:
> > +		/* Use Test{Set,Clear} to only un/charge the memcg once. */
> > +		if (val > 0) {
> > +			if (TestSetPageCgroupFileDirty(pc))
> > +				val = 0;
> > +		} else {
> > +			if (!TestClearPageCgroupFileDirty(pc))
> > +				val = 0;
> > +		}
> 
> I'm wondering why TestSet/TestClear and even the cgroup page flags for
> dirty/writeback/unstable pages are necessary at all (it helps to
> document in changelog if there are any). For example, VFS will call
> TestSetPageDirty() before calling
> mem_cgroup_inc_page_stat(MEMCG_NR_FILE_DIRTY), so there should be no
> chance of false double counting.
> 

1. flag is necessary for moving accounting information between cgroups
   when account_move() occurs.

2. TestSet... is required because there are always race with page_cgroup_lock()'s
   lock bit.

Thanks,
-Kame

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