linux-kernel - Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup

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Message-ID: <20130322105616.GK31457@dhcp22.suse.cz>
Date:	Fri, 22 Mar 2013 11:56:16 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()

On Fri 22-03-13 14:25:23, Glauber Costa wrote:
> On 03/22/2013 02:06 PM, Michal Hocko wrote:
> > On Fri 22-03-13 14:03:30, Glauber Costa wrote:
> >> On 03/22/2013 01:48 PM, Michal Hocko wrote:
> >>> On Fri 22-03-13 13:41:40, Glauber Costa wrote:
> >>>> On 03/22/2013 01:31 PM, Michal Hocko wrote:
> >>>>> On Fri 22-03-13 12:22:23, Glauber Costa wrote:
> >>>>>> On 03/22/2013 12:17 PM, Li Zefan wrote:
> >>>>>>>> GFP_TEMPORARY groups short lived allocations but the mem cache is not
> >>>>>>>>> an ideal candidate of this type of allocations..
> >>>>>>>>>
> >>>>>>> I'm not sure I'm following you...
> >>>>>>>
> >>>>>>> char *memcg_cache_name()
> >>>>>>> {
> >>>>>>> 	char *name = alloc();
> >>>>>>> 	return name;
> >>>>>>> }
> >>>>>>>
> >>>>>>> kmem_cache_dup()
> >>>>>>> {
> >>>>>>> 	name = memcg_cache_name();
> >>>>>>> 	kmem_cache_create_memcg(name);
> >>>>>>> 	free(name);
> >>>>>>> }
> >>>>>>>
> >>>>>>> Isn't this a short lived allocation?
> >>>>>>>
> >>>>>>
> >>>>>> Hi,
> >>>>>>
> >>>>>> Thanks for identifying and fixing this.
> >>>>>>
> >>>>>> Li is right. The cache name will live long, but this is because the
> >>>>>> slab/slub caches will strdup it internally. So the actual memcg
> >>>>>> allocation is short lived.
> >>>>>
> >>>>> OK, I have totally missed that. Sorry about the confusion. Then all the
> >>>>> churn around the allocation is pointless, no?
> >>>>> What about:
> >>>>
> >>>> If we're really not concerned about stack, then yes. Even if always
> >>>> running from workqueues, a PAGE_SIZEd stack variable seems risky to me.
> >>>
> >>> This is not on stack. It is static
> >>>
> >> Ah, right, I totally missed that. And then you're taking the mutex.
> >>
> >> But actually, you don't need to take the mutex. All calls to
> >> kmem_cache_dup are protected by the memcg_cache_mutex.
> > 
> > Yes and I am not taking that mutex. I've just added lockdep assert to
> > make sure that this still holds true.
> > 
> It is impressive what a busy week does to our brains...

Tell me something about that.

> I read the code as lockdep_assert(memcg_cache_mutex), and then later on
> mutex_lock(&memcg_mutex). But reading again, that was a just an
> rcu_read_lock(). Good thing it is Friday
> 
> You guys can add my Acked-by, and thanks again

Li, are you ok to take the page via your tree?

-- 
Michal Hocko
SUSE Labs
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