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

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Date:	Tue, 9 Apr 2013 11:53:50 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Li Zefan <lizefan@...wei.com>
CC:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.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 07:18 AM, 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.
> 
I feel more comfortable with the check as well. This is a debugging
interface after all. People tend to resort to this when things go wrong.
So at this point the less assumptions we make, the better.

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