linux-kernel - Re: [PATCH 13/12] memcg: don't need memcg->memcg

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Date:	Mon, 8 Apr 2013 16:25:03 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Li Zefan <lizefan@...wei.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>,
	Glauber Costa <glommer@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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

On Mon 08-04-13 14:36:52, Li Zefan wrote:
[...]
> @@ -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;

The interface is only for debugging, all right, but that doesn't mean we
should allocate a buffer for each read. Why cannot we simply use
cgroup_path for seq_printf directly? Can we still race with the group
rename?

> +
> +	/*
> +	 * 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);
>  
> @@ -5203,6 +5185,7 @@ static int mem_cgroup_dangling_read(struct cgroup *cont, struct cftype *cft,
>  	}
>  
>  	mutex_unlock(&dangling_memcgs_mutex);
> +	free_pages((unsigned long)memcg_name, 0);
>  	return 0;
>  }
>  #endif
> -- 
> 1.8.0.2
> --
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-- 
Michal Hocko
SUSE Labs
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