[Devel] Re: Question : memrlimit cgroup's task_move (2.6.26-rc5-mm3)

[Balbir Singh]

KAMEZAWA Hiroyuki wrote:
> On Thu, 19 Jun 2008 23:55:56 +0530
> Balbir Singh <balbir at linux.vnet.ibm.com> wrote:
> 
>> * KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> [2008-06-19 12:14:35]:
>>
>>> I used memrlimit cgroup at the first time.
>>>
>>> May I ask a question about memrlimit cgroup ?
>>>
>> Hi, Kamezawa-San,
>>
>> Could you please review/test the patch below to see if it solves your
>> problem? If it does, I'll push it up to Andrew
>>
> 
> At quick glance,
>> +	/*
>> +	 * NOTE: Even though we do the necessary checks in can_attach(),
>> +	 * by the time we come here, there is a chance that we still
>> +	 * fail (the memrlimit cgroup has grown its usage, and the
>> +	 * addition of total_vm will no longer fit into its limit)
>> +	 */
> I don't like this kind of holes. Considering tests which are usually done
> by developpers, the problem seems not to be mentioned as "rare"..
> It seems we can easily cause Warning. right ?
> 
> Even if you don't want to handle this case now, please mention as "TBD" 
> rather than as "NOTE".
> 

Honestly to fix this problem completely, we need transactional management in
cgroups. Both can_attach() and attach() are called with cgroup_mutex held, but
total_vm is changed with mmap_sem held.

What we can do is

1. Implement a routine attach_failed() in cgroups, that is called for each task
   for which can_attach() succeeded, if any of the can_attach() routine returns
   an error
2. Do the migration in can_attach() and unroll in attach_failed()



-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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