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Date: Fri, 29 Oct 2010 19:13:00 +0800
From: Wu Fengguang <fengguang.wu@intel.com>
To: Greg Thelen <gthelen@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "containers@lists.osdl.org" <containers@lists.osdl.org>,
        Andrea Righi <arighi@develer.com>,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
        Minchan Kim <minchan.kim@gmail.com>,
        Ciju Rajan K <ciju@linux.vnet.ibm.com>,
        David Rientjes <rientjes@google.com>
Subject: Re: [PATCH v4 06/11] memcg: add dirty page accounting
 infrastructure
Message-ID: <20101029111300.GB29774@localhost>
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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.

Thanks,
Fengguang