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

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Date:	Fri, 31 Jan 2014 15:29:36 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	David Rientjes <rientjes@...gle.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 01/31/2014 02:42 PM, David Rientjes wrote:
> 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?

Well, I mean not the approach - here everything is right - but how it
looks. This

len += 4
len += 5

looks scary even with comments, IMHO. Note, I do not stand for this
temporary buffer - it was introduced long before I started tweaking this
code. I just want to say that substituting it now with something (OK,
less, but IMHO still) cumbersome is not a good idea provided soon it
will be possible to remove tmp_name while still having the code looking
nice. If you insist, I don't mind, but... why?

Thanks.
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