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

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Date:	Wed, 27 Mar 2013 16:32:20 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>,
	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 19:19:58, Glauber Costa wrote:
> On 03/27/2013 07:11 PM, Michal Hocko wrote:
> > 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.
> 
> theoretically, this is PATH_MAX + max cache name.

So do you prefer kmalloc(PATH_MAX) or the page allocator directly as
Johannes suggests? I agree tha kamlloc(PAGE_SIZE) looks weird.

-- 
Michal Hocko
SUSE Labs
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