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Date:	Tue, 09 Apr 2013 12:10:03 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Li Zefan <lizefan@...wei.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>,
	Glauber Costa <glommer@...allels.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 13/12] memcg: don't need memcg->memcg_name

(2013/04/08 15:36), Li Zefan wrote:
> Now memcg has the same life cycle as its corresponding cgroup,
> we don't have to save the cgroup path name in memcg->memcg_name.
> 
> Signed-off-by: Li Zefan <lizefan@...wei.com>
> ---
>   mm/memcontrol.c | 65 +++++++++++++++++++++------------------------------------
>   1 file changed, 24 insertions(+), 41 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index aeab1d3..06e995e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -306,20 +306,12 @@ struct mem_cgroup {
>   		struct list_head dead;
>   	};
>   
> -	union {
> -		/*
> -		 * Should we move charges of a task when a task is moved into
> -		 * this mem_cgroup ? And what type of charges should we move ?
> -		 */
> -		unsigned long move_charge_at_immigrate;
> +	/*
> +	 * Should we move charges of a task when a task is moved into
> +	 * this mem_cgroup ? And what type of charges should we move ?
> +	 */
> +	unsigned long move_charge_at_immigrate;
>   
> -		/*
> -		 * We are no longer concerned about moving charges after memcg
> -		 * is dead. So we will fill this up with its name, to aid
> -		 * debugging.
> -		 */
> -		char *memcg_name;
> -	};
>   	/*
>   	 * set > 0 if pages under this cgroup are moving to other cgroup.
>   	 */
> @@ -381,36 +373,10 @@ static inline void memcg_dangling_free(struct mem_cgroup *memcg)
>   	mutex_lock(&dangling_memcgs_mutex);
>   	list_del(&memcg->dead);
>   	mutex_unlock(&dangling_memcgs_mutex);
> -	free_pages((unsigned long)memcg->memcg_name, 0);
>   }
>   
>   static inline void memcg_dangling_add(struct mem_cgroup *memcg)
>   {
> -	/*
> -	 * cgroup.c will do page-sized allocations most of the time,
> -	 * so we'll just follow the pattern. Also, __get_free_pages
> -	 * is a better interface than kmalloc for us here, because
> -	 * we'd like this memory to be always billed to the root cgroup,
> -	 * not to the process removing the memcg. While kmalloc would
> -	 * require us to wrap it into memcg_stop/resume_kmem_account,
> -	 * with __get_free_pages we just don't pass the memcg flag.
> -	 */
> -	memcg->memcg_name = (char *)__get_free_pages(GFP_KERNEL, 0);
> -
> -	/*
> -	 * we will, in general, just ignore failures. No need to go crazy,
> -	 * being this just a debugging interface. It is nice to copy a memcg
> -	 * name over, but if we (unlikely) can't, just the address will do
> -	 */
> -	if (!memcg->memcg_name)
> -		goto add_list;
> -
> -	if (cgroup_path(memcg->css.cgroup, memcg->memcg_name, PAGE_SIZE) < 0) {
> -		free_pages((unsigned long)memcg->memcg_name, 0);
> -		memcg->memcg_name = NULL;
> -	}
> -
> -add_list:
>   	INIT_LIST_HEAD(&memcg->dead);
>   	mutex_lock(&dangling_memcgs_mutex);
>   	list_add(&memcg->dead, &dangling_memcgs);
> @@ -5188,12 +5154,28 @@ static int mem_cgroup_dangling_read(struct cgroup *cont, struct cftype *cft,
>   					struct seq_file *m)
>   {
>   	struct mem_cgroup *memcg;
> +	char *memcg_name;
> +	int ret;
> +
> +	/*
> +	 * cgroup.c will do page-sized allocations most of the time,
> +	 * so we'll just follow the pattern. Also, __get_free_pages
> +	 * is a better interface than kmalloc for us here, because
> +	 * we'd like this memory to be always billed to the root cgroup,
> +	 * not to the process removing the memcg. While kmalloc would
> +	 * require us to wrap it into memcg_stop/resume_kmem_account,
> +	 * with __get_free_pages we just don't pass the memcg flag.
> +	 */
> +	memcg_name = (char *)__get_free_pages(GFP_KERNEL, 0);
> +	if (!memcg_name)
> +		return -ENOMEM;
>   
>   	mutex_lock(&dangling_memcgs_mutex);
>   
>   	list_for_each_entry(memcg, &dangling_memcgs, dead) {
> -		if (memcg->memcg_name)
> -			seq_printf(m, "%s:\n", memcg->memcg_name);
> +		ret = cgroup_path(memcg->css.cgroup, memcg_name, PAGE_SIZE);
> +		if (!ret)
> +			seq_printf(m, "%s:\n", memcg_name);
>   		else
>   			seq_printf(m, "%p (name lost):\n", memcg);
>   

I'm sorry for dawm question ...when this error happens ?
We may get ENAMETOOLONG even with PAGE_SIZE(>=4096bytes) buffer ?

Thanks,
-Kame





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