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

[Li Zefan]

>> Thanks for identifying and fixing this.
>>
>> Li is right. The cache name will live long, but this is because the
>> slab/slub caches will strdup it internally. So the actual memcg
>> allocation is short lived.
> 
> OK, I have totally missed that. Sorry about the confusion. Then all the
> churn around the allocation is pointless, no?
> What about:
> ---
>>From 7ed7f53bb597e8cb40d9ac91ce16142fb60f1e93 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxx>
> Date: Fri, 22 Mar 2013 10:22:54 +0100
> Subject: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()
> 
> As cgroup supports rename, it's unsafe to dereference dentry->d_name
> without proper vfs locks. Fix this by using cgroup_name() rather than
> dentry directly.
> 
> Also open code memcg_cache_name because it is called only from
> kmem_cache_dup which frees the returned name right after
> kmem_cache_create_memcg makes a copy of it. Such a short-lived
> allocation doesn't make too much sense. So replace it by a static
> buffer as kmem_cache_dup is called with memcg_cache_mutex.
> 

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.

> Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> ---
>  mm/memcontrol.c |   33 +++++++++++----------------------
>  1 file changed, 11 insertions(+), 22 deletions(-)
...
>  static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
>  					 struct kmem_cache *s)
>  {
>  	char *name;
>  	struct kmem_cache *new;
> +	static char tmp_name[PAGE_SIZE];
>  
> -	name = memcg_cache_name(memcg, s);
> -	if (!name)
> -		return NULL;
> +	lockdep_assert_held(&memcg_cache_mutex);
> +
> +	rcu_read_lock();
> +	tmp_name = snprintf(tmp_name, sizeof(tmp_name), "%s(%d:%s)", s->name,
> +			 memcg_cache_id(memcg), cgroup_name(memcg->css.cgroup));

I guess you didn't turn on CONFIG_MEMCG_KMEM?

snprintf() returns a int value.

> +	rcu_read_unlock();
>  
> -	new = kmem_cache_create_memcg(memcg, name, s->object_size, s->align,
> +	new = kmem_cache_create_memcg(memcg, tmp_name, s->object_size, s->align,
>  				      (s->flags & ~SLAB_PANIC), s->ctor, s);
>  
>  	if (new)
>  		new->allocflags |= __GFP_KMEMCG;
>  
> -	kfree(name);
>  	return new;
>  }
>  
> 

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