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Date: Tue, 28 Feb 2012 14:37:38 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Fengguang Wu <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>, Jan Kara <jack@suse.cz>,
        Ying Han <yinghan@google.com>,
        "hannes@cmpxchg.org" <hannes@cmpxchg.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Rik van Riel <riel@redhat.com>, Andrea Righi <andrea@betterlinux.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/9] memcg: add dirty page accounting infrastructure
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	<20120228144746.971869014@intel.com>
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On Tue, 28 Feb 2012 22:00:24 +0800
Fengguang Wu <fengguang.wu@intel.com> wrote:

> From: Greg Thelen <gthelen@google.com>
> 
> Add memcg routines to count dirty, writeback, and unstable_NFS pages.
> These routines are not yet used by the kernel to count such pages.  A
> later change adds kernel calls to these new routines.
> 
> As inode pages are marked dirty, if the dirtied page's cgroup differs
> from the inode's cgroup, then mark the inode shared across several
> cgroup.
> 
> ...
>
> @@ -1885,6 +1888,44 @@ void mem_cgroup_update_page_stat(struct 
>  			ClearPageCgroupFileMapped(pc);
>  		idx = MEM_CGROUP_STAT_FILE_MAPPED;
>  		break;
> +
> +	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;
> +		}

Made me scratch my head for a while, but I see now that the `val' arg
to (the undocumented) mem_cgroup_update_page_stat() can only ever have
the values 1 or -1.  I hope.

>
> ...
>