linux-kernel - Re: [PATCH 2/5] memcg: quick memcg lookup array

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Date:	Mon, 30 Aug 2010 17:03:24 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	gthelen@...gle.com, m-ikeda@...jp.nec.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"menage@...gle.com" <menage@...gle.com>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH 2/5] memcg: quick memcg lookup array

> Index: mmotm-0811/mm/memcontrol.c
> ===================================================================
> --- mmotm-0811.orig/mm/memcontrol.c
> +++ mmotm-0811/mm/memcontrol.c
> @@ -195,6 +195,7 @@ static void mem_cgroup_oom_notify(struct
>   */
>  struct mem_cgroup {
>  	struct cgroup_subsys_state css;
> +	int	valid; /* for checking validness under RCU access.*/
>  	/*
>  	 * the counter to account for memory usage
>  	 */
Do we really need to add this new member ?
Can't we safely access "mem(=rcu_dereference(mem_cgroup[id]))" under rcu_read_lock() ?
(iow, "mem" is not freed ?)


> @@ -4049,6 +4068,7 @@ static void __mem_cgroup_free(struct mem
>  	mem_cgroup_remove_from_trees(mem);
>  	free_css_id(&mem_cgroup_subsys, &mem->css);
>  
> +	atomic_dec(&mem_cgroup_num);
>  	for_each_node_state(node, N_POSSIBLE)
>  		free_mem_cgroup_per_zone_info(mem, node);
>  
> @@ -4059,6 +4079,19 @@ static void __mem_cgroup_free(struct mem
>  		vfree(mem);
>  }
>  
> +static void mem_cgroup_free(struct mem_cgroup *mem)
> +{
> +	/* No more lookup */
> +	mem->valid = 0;
> +	rcu_assign_pointer(mem_cgroups[css_id(&mem->css)], NULL);
> +	/*
> +	 * Because we call vfree() etc...use synchronize_rcu() rather than
> + 	 * call_rcu();
> + 	 */
> +	synchronize_rcu();
> +	__mem_cgroup_free(mem);
> +}
> +
>  static void mem_cgroup_get(struct mem_cgroup *mem)
>  {
>  	atomic_inc(&mem->refcnt);
> @@ -4068,7 +4101,7 @@ static void __mem_cgroup_put(struct mem_
>  {
>  	if (atomic_sub_and_test(count, &mem->refcnt)) {
>  		struct mem_cgroup *parent = parent_mem_cgroup(mem);
> -		__mem_cgroup_free(mem);
> +		mem_cgroup_free(mem);
>  		if (parent)
>  			mem_cgroup_put(parent);
>  	}
> @@ -4189,9 +4222,11 @@ mem_cgroup_create(struct cgroup_subsys *
>  	atomic_set(&mem->refcnt, 1);
>  	mem->move_charge_at_immigrate = 0;
>  	mutex_init(&mem->thresholds_lock);
> +	atomic_inc(&mem_cgroup_num);
> +	register_memcg_id(mem);
>  	return &mem->css;
>  free_out:
> -	__mem_cgroup_free(mem);
> +	mem_cgroup_free(mem);
>  	root_mem_cgroup = NULL;
>  	return ERR_PTR(error);
>  }
I think mem_cgroup_num should be increased at mem_cgroup_alloc(), because it
is decreased at __mem_cgroup_free(). Otherwise, it can be decreased while it
has not been increased, if mem_cgroup_create() fails after mem_cgroup_alloc().


Thanks,
Daisuke Nishimura.
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