From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with SMTP id 8287C6B024D for ; Thu, 22 Jul 2010 09:16:51 -0400 (EDT) Message-ID: <4C4844BC.4090709@redhat.com> Date: Thu, 22 Jul 2010 09:16:44 -0400 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [BUGFIX][PATCH] Fix false positive BUG_ON in __page_set_anon_rmap References: <20100722164118.d500b850.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20100722164118.d500b850.kamezawa.hiroyu@jp.fujitsu.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: KAMEZAWA Hiroyuki Cc: "akpm@linux-foundation.org" , "linux-mm@kvack.org" , kosaki.motohiro@jp.fujitsu.com, "linux-kernel@vger.kernel.org" , Andrea Arcangeli List-ID: 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. Acked-by: Rik van Riel > Index: mmotm-2.6.35-0719/mm/rmap.c > =================================================================== > --- mmotm-2.6.35-0719.orig/mm/rmap.c > +++ mmotm-2.6.35-0719/mm/rmap.c > @@ -783,8 +783,16 @@ static void __page_set_anon_rmap(struct > if (PageAnon(page)) > return; > anon_vma = anon_vma->root; > - } else > - BUG_ON(PageAnon(page)); > + } else { > + /* > + * In this case, swapped-out-but-not-discarded swap-cache > + * is remapped. So, no need to update page->mapping here. > + * We convice anon_vma poitned by page->mapping is not obsolete > + * because vma->anon_vma is necessary to be a family of it. > + */ > + if (PageAnon(page)) > + return; > + } > > anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; > page->mapping = (struct address_space *) anon_vma; > -- All rights reversed -- 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