linux-kernel - Re: [patch 4/4] memcg: use native word page statistics counters

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Date:	Mon, 8 Nov 2010 09:07:35 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Greg Thelen <gthelen@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <hidave.darkstar@...il.com>,
	Andrea Righi <arighi@...eler.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Wu Fengguang <fengguang.wu@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 4/4] memcg: use native word page statistics counters

On Mon, Nov 8, 2010 at 7:14 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> The statistic counters are in units of pages, there is no reason to
> make them 64-bit wide on 32-bit machines.
>
> Make them native words.  Since they are signed, this leaves 31 bit on
> 32-bit machines, which can represent roughly 8TB assuming a page size
> of 4k.
>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> ---
>  include/linux/memcontrol.h |    2 +-
>  mm/memcontrol.c            |   43 +++++++++++++++++++++----------------------
>  mm/page-writeback.c        |    4 ++--
>  3 files changed, 24 insertions(+), 25 deletions(-)
>

<snip>

>
>  static unsigned long dirty_writeback_pages(void)
>  {
> -       s64 ret;
> +       unsigned long ret;
>
>        ret = mem_cgroup_page_stat(MEMCG_NR_DIRTY_WRITEBACK_PAGES);
> -       if (ret < 0)
> +       if ((long)ret < 0)
>                ret = global_page_state(NR_UNSTABLE_NFS) +
>                        global_page_state(NR_WRITEBACK);

BTW, let me ask a question.
dirty_writeback_pages seems to be depends on mem_cgroup_page_stat's
result(ie, negative) for separate global and memcg.
But mem_cgroup_page_stat could return negative value by per-cpu as
well as root cgroup.
If I understand right, Isn't it a problem?

-- 
Kind regards,
Minchan Kim
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