linux-kernel - Re: [PATCH] memcg: fix mutex not unlocked on memcg_create_kmem

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Date:	Fri, 31 Jan 2014 02:42:25 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>, mhocko@...e.cz,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: fix mutex not unlocked on memcg_create_kmem_cache
 fail path

On Fri, 31 Jan 2014, Vladimir Davydov wrote:

> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -637,6 +637,9 @@ int memcg_limited_groups_array_size;
> >   * better kept as an internal representation in cgroup.c. In any case, the
> >   * cgrp_id space is not getting any smaller, and we don't have to necessarily
> >   * increase ours as well if it increases.
> > + *
> > + * Updates to MAX_SIZE should update the space for the memcg name in
> > + * memcg_create_kmem_cache().
> >   */
> >  #define MEMCG_CACHES_MIN_SIZE 4
> >  #define MEMCG_CACHES_MAX_SIZE MEM_CGROUP_ID_MAX
> > @@ -3400,8 +3403,10 @@ void mem_cgroup_destroy_cache(struct kmem_cache *cachep)
> >  static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
> >  						  struct kmem_cache *s)
> >  {
> > -	char *name = NULL;
> >  	struct kmem_cache *new;
> > +	const char *cgrp_name;
> > +	char *name = NULL;
> > +	size_t len;
> >  
> >  	BUG_ON(!memcg_can_account_kmem(memcg));
> >  
> > @@ -3409,9 +3414,22 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
> >  	if (unlikely(!name))
> >  		return NULL;
> >  
> > +	/*
> > +	 * Format of a memcg's kmem cache name:
> > +	 * <cache-name>(<memcg-id>:<cgroup-name>)
> > +	 */
> > +	len = strlen(s->name);
> > +	/* Space for parentheses, colon, terminator */
> > +	len += 4;
> > +	/* MEMCG_CACHES_MAX_SIZE is USHRT_MAX */
> > +	len += 5;
> > +	BUILD_BUG_ON(MEMCG_CACHES_MAX_SIZE > USHRT_MAX);
> > +
> 
> This looks cumbersome, IMO. Let's leave it as is for now. AFAIK,
> cgroup_name() will be reworked soon so that it won't require RCU-context
> (https://lkml.org/lkml/2014/1/28/530). Therefore, it will be possible to
> get rid of this pointless tmp_name allocation by making
> kmem_cache_create_memcg() take not just name, but printf-like format +
> vargs.
> 

You believe it's less cumbersome to do two memory allocations to figure 
out how much memory you really need to allocate rather than just 
calculating the necessary size?
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