From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id EADC26B015C for ; Mon, 15 Mar 2010 08:55:17 -0400 (EDT) Received: by vws4 with SMTP id 4so742505vws.14 for ; Mon, 15 Mar 2010 05:55:16 -0700 (PDT) Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages From: Minchan Kim In-Reply-To: <20100315112829.GI18274@csn.ul.ie> References: <1268412087-13536-1-git-send-email-mel@csn.ul.ie> <1268412087-13536-3-git-send-email-mel@csn.ul.ie> <28c262361003141728g4aa40901hb040144c5a4aeeed@mail.gmail.com> <20100315143420.6ec3bdf9.kamezawa.hiroyu@jp.fujitsu.com> <20100315112829.GI18274@csn.ul.ie> Content-Type: text/plain; charset="UTF-8" Date: Mon, 15 Mar 2010 21:48:49 +0900 Message-ID: <1268657329.1889.4.camel@barrios-desktop> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Mel Gorman Cc: KAMEZAWA Hiroyuki , Andrew Morton , Andrea Arcangeli , Christoph Lameter , Adam Litke , Avi Kivity , David Rientjes , KOSAKI Motohiro , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: On Mon, 2010-03-15 at 11:28 +0000, Mel Gorman wrote: > The use after free looks like > > 1. page_mapcount(page) was zero so anon_vma was no longer reliable > 2. rcu lock taken but the anon_vma at this point can already be garbage because the > process exited > 3. call try_to_unmap, looks up tha anon_vma and locks it. This causes problems > > I thought the race would be closed but there is still a very tiny window there all > right. The following alternative should close it. What do you think? > > if (PageAnon(page)) { > rcu_read_lock(); > > /* > * If the page has no mappings any more, just bail. An > * unmapped anon page is likely to be freed soon but worse, > * it's possible its anon_vma disappeared between when > * the page was isolated and when we reached here while > * the RCU lock was not held > */ > if (!page_mapcount(page)) { > rcu_read_unlock(); > goto uncharge; > } > > rcu_locked = 1; > anon_vma = page_anon_vma(page); > atomic_inc(&anon_vma->external_refcount); > } > > The rcu_unlock label is not used here because the reference counts were not taken in > the case where page_mapcount == 0. > Looks good to me. Please, repost above code with your use-after-free scenario comment. -- Kind regards, Minchan Kim -- 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