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From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
	Balbir Singh <balbir@linux.vnet.ibm.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [patch 1/5] memcg: no uncharged pages reach page_cgroup_zoneinfo
Date: Fri, 4 Feb 2011 18:23:21 +0900	[thread overview]
Message-ID: <20110204182321.5ba58fdd.kamezawa.hiroyu@jp.fujitsu.com> (raw)
In-Reply-To: <20110204092650.GB2289@cmpxchg.org>

On Fri, 4 Feb 2011 10:26:50 +0100
Johannes Weiner <hannes@cmpxchg.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@cmpxchg.org> wrote:
> > 
> > > All callsites check PCG_USED before passing pc->mem_cgroup, so the
> > > latter is never NULL.
> > > 
> > > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > 
> > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.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@cmpxchg.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;
>  }
>  
> 


  reply	other threads:[~2011-02-04  9:29 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-02-03 14:26 memcg: save 20% of per-page memcg memory overhead Johannes Weiner
2011-02-03 14:26 ` [patch 1/5] memcg: no uncharged pages reach page_cgroup_zoneinfo Johannes Weiner
2011-02-04  0:01   ` KAMEZAWA Hiroyuki
2011-02-04  9:26     ` Johannes Weiner
2011-02-04  9:23       ` KAMEZAWA Hiroyuki [this message]
2011-02-04  4:16   ` Balbir Singh
2011-02-03 14:26 ` [patch 2/5] memcg: change page_cgroup_zoneinfo signature Johannes Weiner
2011-02-04  0:03   ` KAMEZAWA Hiroyuki
2011-02-03 14:26 ` [patch 3/5] memcg: fold __mem_cgroup_move_account into caller Johannes Weiner
2011-02-04  0:07   ` KAMEZAWA Hiroyuki
2011-02-04  0:53     ` Daisuke Nishimura
2011-02-03 14:26 ` [patch 4/5] memcg: condense page_cgroup-to-page lookup points Johannes Weiner
2011-02-04  0:10   ` KAMEZAWA Hiroyuki
2011-02-03 14:26 ` [patch 5/5] memcg: remove direct page_cgroup-to-page pointer Johannes Weiner
2011-02-04  0:19   ` KAMEZAWA Hiroyuki
2011-02-04  9:51     ` Johannes Weiner
2011-02-03 15:02 ` memcg: save 20% of per-page memcg memory overhead Balbir Singh

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