From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752758Ab0DIDQt (ORCPT ); Thu, 8 Apr 2010 23:16:49 -0400 Received: from cantor2.suse.de ([195.135.220.15]:44022 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751974Ab0DIDQr (ORCPT ); Thu, 8 Apr 2010 23:16:47 -0400 Date: Fri, 9 Apr 2010 13:16:41 +1000 From: Nick Piggin To: Peter Zijlstra Cc: Andrea Arcangeli , Avi Kivity , Thomas Gleixner , Rik van Riel , Ingo Molnar , akpm@linux-foundation.org, Linus Torvalds , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Benjamin Herrenschmidt , David Miller , Hugh Dickins , Mel Gorman Subject: Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma() Message-ID: <20100409031641.GG5683@laptop> References: <20100408191737.296180458@chello.nl> <20100408192722.687144862@chello.nl> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100408192722.687144862@chello.nl> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 08, 2010 at 09:17:39PM +0200, Peter Zijlstra wrote: > There is nothing preventing the anon_vma from being detached while we > are spinning to acquire the lock. Most (all?) current users end up > calling something like vma_address(page, vma) on it, which has a > fairly good chance of weeding out wonky vmas. > > However suppose the anon_vma got freed and re-used while we were > waiting to acquire the lock, and the new anon_vma fits with the > page->index (because that is the only thing vma_address() uses to > determine if the page fits in a particular vma, we could end up > traversing faulty anon_vma chains. > > Close this hole for good by re-validating that page->mapping still > holds the very same anon_vma pointer after we acquire the lock, if not > be utterly paranoid and retry the whole operation (which will very > likely bail, because it's unlikely the page got attached to a different > anon_vma in the meantime). Hm, looks like a bugfix? How was this supposed to be safe? > Signed-off-by: Peter Zijlstra > Cc: Hugh Dickins > Cc: Linus Torvalds > --- > mm/rmap.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > Index: linux-2.6/mm/rmap.c > =================================================================== > --- linux-2.6.orig/mm/rmap.c > +++ linux-2.6/mm/rmap.c > @@ -294,6 +294,7 @@ struct anon_vma *page_lock_anon_vma(stru > unsigned long anon_mapping; > > rcu_read_lock(); > +again: > anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping); > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > goto out; > @@ -302,6 +303,12 @@ struct anon_vma *page_lock_anon_vma(stru > > anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); > spin_lock(&anon_vma->lock); > + > + if (page_rmapping(page) != anon_vma) { very unlikely()? > + spin_unlock(&anon_vma->lock); > + goto again; > + } > + > return anon_vma; > out: > rcu_read_unlock(); >