linux-kernel - Re: [RFC][PATCH] Reduce size of swap

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Message-Id: <20090223145828.d14ff015.nishimura@mxp.nes.nec.co.jp>
Date:	Mon, 23 Feb 2009 14:58:28 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	nishimura@....nes.nec.co.jp
Subject: Re: [RFC][PATCH] Reduce size of swap_cgroup by CSS ID v2

I'm sorry for my late reply.

It looks good basically, but I have 1 comment.

>  static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
>  {
> -	struct mem_cgroup *mem;
> +	unsigned short id;
> +	struct mem_cgroup *mem = NULL;
>  	swp_entry_t ent;
>  
>  	if (!PageSwapCache(page))
>  		return NULL;
>  
>  	ent.val = page_private(page);
> -	mem = lookup_swap_cgroup(ent);
> -	if (!mem)
> -		return NULL;
> +	id = lookup_swap_cgroup(ent);
> +	rcu_read_lock();
> +	mem = mem_cgroup_lookup(id);
>  	if (!css_tryget(&mem->css))
We should check whether "mem" is NULL or not before css_tryget, because
"mem" can be NULL(or "id" can be 0) if the page is on swapcache,
that is, remove_from_swap_cache has not been called yet.

Actually, I got NULL pointer dereference bug here.

> -		return NULL;
> +		mem = NULL;
> +	rcu_read_unlock();
>  	return mem;
>  }
>  


Thanks,
Daisuke Nishimura.
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