linux-kernel - Re: [PATCH] memcg: use ID in page

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Date:	Tue, 24 Aug 2010 17:38:39 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	linux-mm@...ck.org,
	"nishimura\@mxp.nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir\@linux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	m-ikeda@...jp.nec.com,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	kamezawa.hiroyuki@...il.com
Subject: Re: [PATCH] memcg: use ID in page_cgroup

On Tue, 24 Aug 2010 01:35:37 -0700
Greg Thelen <gthelen@...gle.com> wrote:

> > rcu_read_lock() is just for delaying to discard object, not for avoiding
> > racy updates. All _updates_ requires proper lock or speculative logic as
> > atomic_inc_not_zero() etc... Basically, RCU is for avoiding use-after-free.
> 
> 
> Thanks for the info.  Referring to your original patch:
> > @@ -2014,11 +2025,11 @@ static int mem_cgroup_move_account(struc
> >  {
> >  	int ret = -EINVAL;
> >  	lock_page_cgroup(pc);
> > -	if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {
> > +	if (PageCgroupUsed(pc) && id_to_memcg(pc->mem_cgroup, true) == from) {
> >  		__mem_cgroup_move_account(pc, from, to, uncharge);
> >  		ret = 0;
> >  	}
> > -	unlock_page_cgroup(pc);
> > +	rcu_read_unlock();
> > 
> 
> It seems like mem_cgroup_move_account() is not balanced.  Why is
> lock_page_cgroup(pc) used to lock but rcu_read_unlock() used to unlock?
> 

Nice catch. It's bug. It seems my eyes were corrupted..
Will be fixed in the next version. Sorry for bad code.

Thanks,
-Kame

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