Subject: Re: [PATCH v7 10/10] Disable task moving when using kernel memory accounting

From: Glauber Costa <[email-masked]>
Date: Fri, 2 Dec 2011 16:11:56 -0200

On 11/30/2011 12:22 AM, KAMEZAWA Hiroyuki wrote:
> On Tue, 29 Nov 2011 21:57:01 -0200
> Glauber Costa<[email-masked]>  wrote:
>
>> Since this code is still experimental, we are leaving the exact
>> details of how to move tasks between cgroups when kernel memory
>> accounting is used as future work.
>>
>> For now, we simply disallow movement if there are any pending
>> accounted memory.
>>
>> Signed-off-by: Glauber Costa<[email-masked]>
>> CC: Hiroyouki Kamezawa<[email-masked]>
>> ---
>>   mm/memcontrol.c |   23 ++++++++++++++++++++++-
>>   1 files changed, 22 insertions(+), 1 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index a31a278..dd9a6d9 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5453,10 +5453,19 @@ static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
>>   {
>>   	int ret = 0;
>>   	struct mem_cgroup *memcg = mem_cgroup_from_cont(cgroup);
>> +	struct mem_cgroup *from = mem_cgroup_from_task(p);
>> +
>> +#if defined(CONFIG_CGROUP_MEM_RES_CTLR_KMEM)&&  defined(CONFIG_INET)
>> +	if (from != memcg&&  !mem_cgroup_is_root(from)&&
>> +	    res_counter_read_u64(&from->tcp_mem.tcp_memory_allocated, RES_USAGE)) {
>> +		printk(KERN_WARNING "Can't move tasks between cgroups: "
>> +			"Kernel memory held.\n");
>> +		return 1;
>> +	}
>> +#endif
>
> I wonder....reading all codes again, this is incorrect check.
>
> Hm, let me cralify. IIUC, in old code, "prevent moving" is because you hold
> reference count of cgroup, which can cause trouble at rmdir() as leaking refcnt.
right.

> BTW, because socket is a shared resource between cgroup, changes in mm->owner
> may cause task cgroup moving implicitly. So, if you allow leak of resource
> here, I guess... you can take mem_cgroup_get() refcnt which is memcg-local and
> allow rmdir(). Then, this limitation may disappear.

Sorry, I didn't fully understand. Can you clarify further?
If the task is implicitly moved, it will end up calling can_attach as 
well, right?

>
> Then, users will be happy but admins will have unseen kernel resource usage in
> not populated(by rmdir) memcg. Hm, big trouble ?
>
> Thanks,
> -Kame
>

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