linux-kernel - Re: [RFC][PATCH] cgroup: fix permanent wait in rmdir

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Date:	Tue, 23 Jun 2009 13:54:05 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"menage@...gle.com" <menage@...gle.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [RFC][PATCH] cgroup: fix permanent wait in rmdir

On Tue, 23 Jun 2009 13:44:20 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> On Tue, 23 Jun 2009 13:13:33 +0900
> Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> 
> > On Tue, 23 Jun 2009 09:22:23 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > > On Mon, 22 Jun 2009 18:37:07 +0900
> > > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > > 
> > > > previous discussion was this => http://marc.info/?t=124478543600001&r=1&w=2
> > > > 
> > > > I think this is a minimum fix (in code size and behavior) and because
> > > > we can take a BIG LOCK, this kind of check is necessary, anyway.
> > > > Any comments are welcome.
> > > 
> > > I'll split this into 2 patches...and I found I should check page-migration, too.
> > I'll wait a new version, but can you explain in advance this page-migration case ?
> > 
> 
> Not far from swap-in case.
> 
> Assume cgroup "A" which includes file caches. A task in other group mmap file caches
> and do page migration and rmdir against "A" is called at the same time.
> 
> In mem_cgroup_prepare_migration(), following check is used.
> 
> ==
> 	lock_page_cgroup(pc);
> 	if (PageCgroupUsed(pc)) {
> 		mem = pc->mem_cgroup;
> 		css_get(&mem->css);
> 	}
> 	unlock_page_cgroup(pc);
> 	<======================================(*)
> 	if (mem) {
> 		<==============================(**)
> 		try_charge();
> 		...
> 	}
> ==
> 
> At (*), we grab css refcnt which can be under pre_destroy() and
> At (**), pre_destroy may returns 0 but charge may be done after the end of pre_destroy().
> 
Ah I see, you're right.

Thank you for your clarification.

Daisuke Nishimura.

> 
> > > > +static int mem_cgroup_retry_rmdir(struct cgroup_subsys *ss,
> > > > +				  struct cgroup *cont)
> > > > +{
> > > > +	struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
> > > > +
> > > > +	if (res_counter_read_u64(&memcg->res, RES_USAGE))
> > It should be &mem->res.
> > 
> yes.
> too many typos in my patches in these days..
> 
> Thanks,
> -Kame
> 
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