From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with SMTP id 3ED996B024D for ; Thu, 22 Jul 2010 13:39:25 -0400 (EDT) Date: Thu, 22 Jul 2010 19:39:20 +0200 From: Andrea Arcangeli Subject: Re: [BUGFIX][PATCH] Fix false positive BUG_ON in __page_set_anon_rmap Message-ID: <20100722173920.GI24928@random.random> References: <20100722164118.d500b850.kamezawa.hiroyu@jp.fujitsu.com> <4C4844BC.4090709@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4C4844BC.4090709@redhat.com> Sender: owner-linux-mm@kvack.org To: Rik van Riel Cc: KAMEZAWA Hiroyuki , "akpm@linux-foundation.org" , "linux-mm@kvack.org" , kosaki.motohiro@jp.fujitsu.com, "linux-kernel@vger.kernel.org" List-ID: On Thu, Jul 22, 2010 at 09:16:44AM -0400, Rik van Riel wrote: > On 07/22/2010 03:41 AM, KAMEZAWA Hiroyuki wrote: > > Rik, how do you think ? > > > > == > > From: KAMEZAWA Hiroyuki > > > > Problem: wrong BUG_ON() in __page_set_anon_rmap(). > > Kernel version: mmotm-0719 > > > Description: > > Even if SwapCache is fully unmapped and mapcount goes down to 0, > > page->mapping is not cleared and will remain on memory until kswapd or some > > finds it. If a thread cause a page fault onto such "unmapped-but-not-discarded" > > swapcache, it will see a swap cache whose mapcount is 0 but page->mapping has a > > valid value. > > > > When it's reused at do_swap_page(), __page_set_anon_rmap() is called with > > "exclusive==1" and hits BUG_ON(). But this BUG_ON() is wrong. Nothing bad > > with rmapping a page which has page->mapping isn't 0. > > Yes, you are absolutely right. > I already noticed the problem when I merged your patch in aa.git (before it would only be exclusive=0 in do_swap_page so it wasn't a false positive), and I fixed it this way: http://git.kernel.org/?p=linux/kernel/git/andrea/aa.git;a=commitdiff;h=2fe4f42f0f17498984b3f86b2339d583004b45de;hp=ffd146080305632406d97c7f6f984a648854d755 So I retained the BUG_ON for the real page_add_anon_rmap. Maybe not worth it but you can have a look at my solution if you're interested to retain it too. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org