linux-kernel - Re: [PATCH 1/2 v2] memcg: make oom_lock 0 and 1 based rather than coutner

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Date:	Wed, 10 Aug 2011 09:22:24 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	Balbir Singh <bsingharora@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] memcg: make oom_lock 0 and 1 based rather than
 coutner

On Tue, 9 Aug 2011 17:37:32 +0200
Johannes Weiner <jweiner@...hat.com> wrote:

> On Tue, Aug 09, 2011 at 05:22:18PM +0200, Michal Hocko wrote:
> > On Tue 09-08-11 16:03:12, Johannes Weiner wrote:
> > >  	struct mem_cgroup *iter, *failed = NULL;
> > >  	bool cond = true;
> > >  
> > >  	for_each_mem_cgroup_tree_cond(iter, mem, cond) {
> > > -		bool locked = iter->oom_lock;
> > > -
> > > -		iter->oom_lock = true;
> > > -		if (lock_count == -1)
> > > -			lock_count = iter->oom_lock;
> > > -		else if (lock_count != locked) {
> > > +		if (iter->oom_lock) {
> > >  			/*
> > >  			 * this subtree of our hierarchy is already locked
> > >  			 * so we cannot give a lock.
> > >  			 */
> > > -			lock_count = 0;
> > >  			failed = iter;
> > >  			cond = false;
> > > -		}
> > > +		} else
> > > +			iter->oom_lock = true;
> > >  	}
> > >  
> > >  	if (!failed)
> > 
> > We can return here and get rid of done label.
> 
> Ah, right you are.  Here is an update.
> 
> ---
> From 86b36904033e6c6a1af4716e9deef13ebd31e64c Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <jweiner@...hat.com>
> Date: Tue, 9 Aug 2011 15:31:30 +0200
> Subject: [patch] memcg: fix hierarchical oom locking
> 
> Commit "79dfdac memcg: make oom_lock 0 and 1 based rather than
> counter" tried to oom lock the hierarchy and roll back upon
> encountering an already locked memcg.
> 
> The code is confused when it comes to detecting a locked memcg,
> though, so it would fail and rollback after locking one memcg and
> encountering an unlocked second one.
> 
> The result is that oom-locking hierarchies fails unconditionally and
> that every oom killer invocation simply goes to sleep on the oom
> waitqueue forever.  The tasks practically hang forever without anyone
> intervening, possibly holding locks that trip up unrelated tasks, too.
> 
> Signed-off-by: Johannes Weiner <jweiner@...hat.com>
> Acked-by: Michal Hocko <mhocko@...e.cz>

Thanks,
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>


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