linux-kernel - Re: [-mm patch] Show memcg information during OOM

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Message-ID: <20090203053358.GO918@balbir.in.ibm.com>
Date:	Tue, 3 Feb 2009 11:03:58 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Li Zefan <lizf@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [-mm patch] Show memcg information during OOM

* David Rientjes <rientjes@...gle.com> [2009-02-02 21:25:52]:

> On Tue, 3 Feb 2009, Balbir Singh wrote:
> 
> > > > > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > > > > > index d3b9bac..b8e53ae 100644
> > > > > > --- a/mm/oom_kill.c
> > > > > > +++ b/mm/oom_kill.c
> > > > > > @@ -392,6 +392,7 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
> > > > > >  			current->comm, gfp_mask, order, current->oomkilladj);
> > > > > >  		task_lock(current);
> > > > > >  		cpuset_print_task_mems_allowed(current);
> > > > > > +		mem_cgroup_print_mem_info(mem);
> > > > > 
> > > > > I think this can be put outside the task lock. The lock is used to call task_cs() safely in
> > > > > cpuset_print_task_mems_allowed().
> > > > >
> > > > 
> > > > Thanks, I'll work on that in the next version.
> > > >  
> > > 
> > > I was also wondering about this and assumed that it was necessary to 
> > > prevent the cgroup from disappearing during the oom.  If task_lock() isn't 
> > > held, is the memcg->css.cgroup->dentry->d_name.name dereference always 
> > > safe without rcu?
> > >
> > 
> > oom_kill_process is called with tasklist_lock held (read-mode). That
> > should suffice, no? The memcg cannot go away since it has other groups
> > or tasks associated with it. 
> > 
> 
> I don't see how this prevents a task from being reattached to a different 
> cgroup and then a rmdir on memcg->css.cgroup would destroy the dentry 
> without cgroup_mutex or dereferencing via rcu.

That scenario is not possible today from the memory controller
perspective.

We hold memcg_tasklist during task movement and during OOM, task
migration is held till OOM completes.

-- 
	Balbir
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