linux-kernel - Re: [Experimental][PATCH 19/21] memcg-fix-pre-destroy.patch

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Message-Id: <20081204184309.da8264c0.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 4 Dec 2008 18:43:09 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	Paul Menage <menage@...gle.com>
Subject: Re: [Experimental][PATCH 19/21] memcg-fix-pre-destroy.patch

On Thu, 4 Dec 2008 18:34:28 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:

> Added CC: Paul Menage <menage@...gle.com>
> 
> > @@ -2096,7 +2112,7 @@ static void mem_cgroup_get(struct mem_cg
> >  static void mem_cgroup_put(struct mem_cgroup *mem)
> >  {
> >  	if (atomic_dec_and_test(&mem->refcnt)) {
> > -		if (!mem->obsolete)
> > +		if (!css_under_removal(&mem->css))
> >  			return;
> >  		mem_cgroup_free(mem);
> >  	}
> I don't think it's safe to check css_under_removal here w/o cgroup_lock.
> (It's safe *NOW* just because memcg is the only user of css->refcnt.)
> 

> As Li said before, css_under_removal doesn't necessarily mean
> this this group has been destroyed, but mem_cgroup will be freed.
> 
> But adding cgroup_lock/unlock here causes another dead lock,
> because mem_cgroup_get_next_node calls mem_cgroup_put.
> 
> hmm.. hierarchical reclaim code will be re-written completely by [21/21],
> so would it be better to change patch order or to take another approach ?
> 
Hmm, ok.

How about this ?
==
	At initlization, mem_cgroup_create(), set memcg->refcnt to be 1.

	At destroy(), put this refcnt by 1.

	remove css_under_removal(&mem->css) check.
==

-Kame

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