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

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Message-ID: <20150526163646.GA29968@redhat.com>
Date:	Tue, 26 May 2015 18:36:46 +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:
>
> @@ -426,17 +426,7 @@ struct mm_struct {
>  	struct kioctx_table __rcu	*ioctx_table;
>  #endif
>  #ifdef CONFIG_MEMCG
> -	/*
> -	 * "owner" points to a task that is regarded as the canonical
> -	 * user/owner of this mm. All of the following must be true in
> -	 * order for it to be changed:
> -	 *
> -	 * current == mm->owner
> -	 * current->mm != mm
> -	 * new_owner->mm == mm
> -	 * new_owner->alloc_lock is held
> -	 */
> -	struct task_struct __rcu *owner;
> +	struct mem_cgroup __rcu *memcg;

Yes, thanks, this is what I tried to suggest ;)

But I can't review this series. Simply because I know nothing about
memcs. I don't even know how to use it.

Just one question,

> +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. Perhaps sock_update_memcg() is fine, but
task_in_mem_cgroup() calls it when find_lock_task_mm() fails, and in
this case ->mm is NULL.

And in fact I can't understand what mem_cgroup_from_task() actually
means, with or without these changes.

And another question. I can't understand what happens when a task
execs... IOW, could you confirm that exec_mmap() does not need
mm_set_memcg(mm, oldmm->memcg) ?

Oleg.

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