linux-kernel - Re: [RFC 3/3] memcg: get rid of mm

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Date:	Tue, 26 May 2015 19:38:22 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 3/3] memcg: get rid of mm_struct::owner

On 05/26, Michal Hocko wrote:
>
> On Tue 26-05-15 18:36:46, Oleg Nesterov wrote:
> >
> > > +static struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
> > > +{
> > > +	if (!p->mm)
> > > +		return NULL;
> > > +	return rcu_dereference(p->mm->memcg);
> > > +}
> >
> > Probably I missed something, but it seems that the callers do not
> > expect it can return NULL.
>
> This hasn't changed by this patch. mem_cgroup_from_task was allowed to
> return NULL even before. I've just made it static because it doesn't
> have any external users anymore.

I see, but it could only return NULL if mem_cgroup_from_css() returns
NULL. Now it returns NULL for sure if the caller is task_in_mem_cgroup(),

	// called when task->mm == NULL

	task_memcg = mem_cgroup_from_task(task);
	css_get(&task_memcg->css);

and this css_get() doesn't look nice if task_memcg == NULL ;)

> I will double check

Yes, please. Perhaps I missed something.

> > And in fact I can't understand what mem_cgroup_from_task() actually
> > means, with or without these changes.
>
> It performs task_struct->mem_cgroup mapping. We cannot use cgroup
> mapping here because the charges are bound to mm_struct rather than
> task.

Sure, this is what I can understand. I meant... OK, lets ignore
"without these changes", because without these changes there are
much more oddities ;) With these changes only ->mm == NULL case
looks unclear.

And btw,

	if (!p->mm)
		return NULL;
	return rcu_dereference(p->mm->memcg);

perhaps this needs a comment. It is not clear what protects ->mm.
But. After this series "p" is always current (if ->mm != NULL), so
this is fine.

Nevermind. Please forget. I feel this needs a bit of cleanup, but
we can always do this later.

Oleg.

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