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

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Date:	Tue, 9 Apr 2013 11:55:07 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Li Zefan <lizefan@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>,
	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 04/09/2013 07:46 AM, Kamezawa Hiroyuki wrote:
> (2013/04/09 12:18), 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;
>>>> +
>>>> +	/*
>>>> +	 * 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 ?
>>>
>>
>> It does no harm to check the return value, and we don't have to
>> worry about if cgroup_path() will be changed to return some other
>> errno like ENOMEM in the future.
>>
> Hmm. but the name is not lost, right ?
> How about returning error rather than making a mixture of lines in different formats ?
> 
I still think it would be preferable to dump as much informative data as
we can. Even if the name is lost, a lot of info can still be present
(from the caches, etc).

The format also should not matter. Nobody will be script-processing this.


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