linux-kernel - Re: [RFC][-mm] Add an owner to the mm

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Message-ID: <47F1D4F3.3040207@linux.vnet.ibm.com>
Date:	Tue, 01 Apr 2008 11:53:47 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Paul Menage <menage@...gle.com>
CC:	Pavel Emelianov <xemul@...nvz.org>,
	Hugh Dickins <hugh@...itas.com>,
	Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, taka@...inux.co.jp,
	linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC][-mm] Add an owner to the mm_struct (v3)

Paul Menage wrote:
> On Mon, Mar 31, 2008 at 10:43 PM, Balbir Singh
> <balbir@...ux.vnet.ibm.com> wrote:
>>  -static struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
>>  +struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
>>   {
>>         return container_of(task_subsys_state(p, mem_cgroup_subsys_id),
>>                                 struct mem_cgroup, css);
>>   }
> 
> This should probably be inlined in the header file if it's needed
> outside this file.

I thought about it, but that also means we need to export struct mem_cgroup into
the header file

>>  +static inline void mm_fork_init_owner(struct task_struct *p)
>>  +{
>>  +}
> 
> I think this is stale.
> 

Yes, it is stale now :)

>>  +
>>  +void mm_update_next_owner(struct mm_struct *mm)
>>  +{
>>  +       struct task_struct *c, *g, *p = current;
>>  +
>>  +       /*
>>  +        * This routine should not be called for init_task
>>  +        */
>>  +       BUG_ON(p == p->parent);
> 
> I think (as you mentioned earlier) that we need an RCU critical
> section in this function, in order for the tasklist traversal to be
> safe.
> 
> Maybe also BUG_ON(p != mm->owner) ?
> 

Yes

>>  +       list_for_each_entry(c, &p->children, sibling) {
>>  +               if (c->mm && (c->mm == mm))
> 
> Since mm != NULL, no need to test for c->mm since if it's NULL then c->mm != mm
> 

OK

>>  +assign_new_owner:
>>  +       BUG_ON(c == p);
>>  +       task_lock(c);
>>  +       if (c->mm != mm) {
>>  +               task_unlock(c);
>>  +               goto retry;
>>  +       }
>>  +       mm->owner = c;
> 
> Here we'll want to call vm_cgroup_update_mm_owner(), to adjust the
> accounting. (Or if in future we end up with more than a couple of
> subsystems that want notification at this time, we'll want to call
> cgroup_update_mm_owner() and have it call any interested subsystems.
> 

I don't think we need to adjust accounting, since only mm->owner is changing and
not the cgroup to which the task/mm belongs. Do we really need to notify? I
don't want to do any notifications under task_lock().

> Paul


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