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

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Message-ID: <20130327151104.GK16579@dhcp22.suse.cz>
Date:	Wed, 27 Mar 2013 16:11:04 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Tejun Heo <tj@...nel.org>, Glauber Costa <glommer@...allels.com>,
	Li Zefan <lizefan@...wei.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()

On Wed 27-03-13 10:58:25, Johannes Weiner wrote:
> On Wed, Mar 27, 2013 at 09:36:39AM +0100, Michal Hocko wrote:
[...]
> > +	/*
> > +	 * kmem_cache_create_memcg duplicates the given name and
> > +	 * cgroup_name for this name requires RCU context.
> > +	 * This static temporary buffer is used to prevent from
> > +	 * pointless shortliving allocation.
> > +	 */
> > +	if (!tmp_name) {
> > +		tmp_name = kmalloc(PAGE_SIZE, GFP_KERNEL);
> > +		WARN_ON_ONCE(!tmp_name);
> 
> Just use the page allocator directly and get a free allocation failure
> warning. 

WARN_ON_ONCE is probably pointless.

> Then again, order-0 pages are considered cheap enough that they never
> even fail in our current implementation.
> 
> Which brings me to my other point: why not just a simple single-page
> allocation?

No objection from me. I was previously thinking about the "proper"
size for something that is a file name. So I originally wanted to use
PATH_MAX instead but ended up with PAGE_SIZE for reasons I do not
remember now.  Maybe we can use NAME_MAX instead. I just do not like to
use page allocator directly when allocatating something like strings
etc...

To be honest, I do not care much which way to go.

> This just seems a little overelaborate.  I think this path would be
> taken predominantly after cgroup creation and fork where we do a bunch
> of allocations anyway.  And it happens asynchroneously from userspace,
> so it's not even really performance critical.

-- 
Michal Hocko
SUSE Labs
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