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

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Date:	Tue, 24 Feb 2009 09:10:34 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
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>
Subject: Re: [RFC][PATCH] Reduce size of swap_cgroup by CSS ID v2

On Mon, 23 Feb 2009 14:58:28 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:

> 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.
> 
Okay, will fix.

Thanks,
-kame


> > -		return NULL;
> > +		mem = NULL;
> > +	rcu_read_unlock();
> >  	return mem;
> >  }
> >  
> 
> 
> Thanks,
> Daisuke Nishimura.
> --
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