Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()

Glauber Costa
2013-03-26T09:02:08Z

On 03/26/2013 12:43 PM, Michal Hocko wrote:
> On Tue 26-03-13 12:35:58, Glauber Costa wrote:
>>>>
>>>> I doubt it's a win to add 4K to kernel text size instead of adding
>>>> a few extra lines of code... but it's up to you.
>>>
>>> I will leave the decision to Glauber. The updated version which uses
>>> kmalloc for the static buffer is bellow.
>>>
>> I prefer to allocate dynamically here. But although I understand why we
>> need to call cgroup_name, I don't understand what is wrong with
>> kasprintf if we're going to allocate anyway. It will allocate a string
>> just big enough. A PAGE_SIZE'd allocation is a lot more likely to fail.
>>
>> Now, if we really want to be smart here, we can do something like what
>> I've done for the slub attribute buffers, that can actually have very
>> long values.
>>
>> allocate a small buffer that will hold 80 % > of the allocations (256
>> bytes should be enough for most cache names), and if the string is
>> bigger than this, we allocate. Once we allocate, we save it in a static
>> pointer and leave it there. The hope here is that we may be able to
>> live without ever allocating in many systems.
>>
>>> +
>>> + /*
>>> + * 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.
>>> + */
>> The comment is also no longer true if you don't resort to a static buffer.
>
> The buffer _is_ static (read global variable hidden with the function
> scope).
>

Although correct, it is a bit misleading. It is static in the sense it
is held by a static variable. But it is acquired by kmalloc...

In any way, this is a tiny detail.

FWIW, I am fine with the patch you provided:

Acked-by: Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>

We could go seeing how big those allocations are in practice, but I
doubt it is worth the trouble.