linux-kernel - Re: [patch 1/5] memcg: no uncharged pages reach page_cgroup

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Date:	Fri, 4 Feb 2011 18:23:21 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/5] memcg: no uncharged pages reach
 page_cgroup_zoneinfo

On Fri, 4 Feb 2011 10:26:50 +0100
Johannes Weiner <hannes@...xchg.org> wrote:

> On Fri, Feb 04, 2011 at 09:01:45AM +0900, KAMEZAWA Hiroyuki wrote:
> > On Thu,  3 Feb 2011 15:26:02 +0100
> > Johannes Weiner <hannes@...xchg.org> wrote:
> > 
> > > All callsites check PCG_USED before passing pc->mem_cgroup, so the
> > > latter is never NULL.
> > > 
> > > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > 
> > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> Thank you!
> 
> > I want BUG_ON() here.
> 
> I thought about it too at first.  But look at the callsites, all but
> one of them do not even expect this function to return NULL, so if
> this condition had ever been true, we would have seen crashes in the
> callsites.
> 

Hmm ok.
-Kame

> The only caller that checks for NULL is
> mem_cgroup_get_reclaim_stat_from_page() and I propose to remove that
> as well; patch attached.
> 
> Do you insist on the BUG_ON?
> 
> ---
> Subject: memcg: page_cgroup_zoneinfo never returns NULL
> 
> For a page charged to a memcg, there is always valid memcg per-zone
> info.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> ---
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4a4483d..5f974b3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1017,9 +1017,6 @@ mem_cgroup_get_reclaim_stat_from_page(struct page *page)
>  	/* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
>  	smp_rmb();
>  	mz = page_cgroup_zoneinfo(pc->mem_cgroup, page);
> -	if (!mz)
> -		return NULL;
> -
>  	return &mz->reclaim_stat;
>  }
>  
> 

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