linux-kernel - Re: [PATCH 3/8] memcg, slab: never try to merge memcg caches

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Message-ID: <20140204151145.GI4890@dhcp22.suse.cz>
Date:	Tue, 4 Feb 2014 16:11:45 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	akpm@...ux-foundation.org, rientjes@...gle.com, penberg@...nel.org,
	cl@...ux.com, glommer@...il.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...nvz.org
Subject: Re: [PATCH 3/8] memcg, slab: never try to merge memcg caches

On Tue 04-02-14 18:59:23, Vladimir Davydov wrote:
> On 02/04/2014 06:52 PM, Michal Hocko wrote:
> > On Sun 02-02-14 20:33:48, Vladimir Davydov wrote:
> >> Suppose we are creating memcg cache A that could be merged with cache B
> >> of the same memcg. Since any memcg cache has the same parameters as its
> >> parent cache, parent caches PA and PB of memcg caches A and B must be
> >> mergeable too. That means PA was merged with PB on creation or vice
> >> versa, i.e. PA = PB. From that it follows that A = B, and we couldn't
> >> even try to create cache B, because it already exists - a contradiction.
> > I cannot tell I understand the above but I am totally not sure about the
> > statement bellow.
> >
> >> So let's remove unused code responsible for merging memcg caches.
> > How come the code was unused? find_mergeable called cache_match_memcg...
> 
> Oh, sorry for misleading comment. I mean the code handling merging of
> per-memcg caches is useless, AFAIU: if we find an alias for a per-memcg
> cache on kmem_cache_create_memcg(), the parent of the found alias must
> be the same as the parent_cache passed to kmem_cache_create_memcg(), but
> if it were so, we would never proceed to the memcg cache creation,
> because the cache we want to create already exists.

I am still not sure I understand this correctly. So the outcome of this
patch is that compatible caches of different memcgs can be merged
together? Sorry if this is a stupid question but I am not that familiar
with this area much I am just seeing that cache_match_memcg goes away
and my understanding of the function is that it should prevent from
different memcg's caches merging.
-- 
Michal Hocko
SUSE Labs
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