From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752688Ab0CRLO4 (ORCPT ); Thu, 18 Mar 2010 07:14:56 -0400 Received: from gir.skynet.ie ([193.1.99.77]:58043 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752351Ab0CRLOz (ORCPT ); Thu, 18 Mar 2010 07:14:55 -0400 Date: Thu, 18 Mar 2010 11:14:36 +0000 From: Mel Gorman To: KOSAKI Motohiro Cc: Minchan Kim , KAMEZAWA Hiroyuki , Andrew Morton , Andrea Arcangeli , Christoph Lameter , Adam Litke , Avi Kivity , David Rientjes , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Message-ID: <20100318111436.GK12388@csn.ul.ie> References: <20100317104734.4C8E.A69D9226@jp.fujitsu.com> <20100317115133.GG12388@csn.ul.ie> <20100318094720.872F.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20100318094720.872F.A69D9226@jp.fujitsu.com> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 18, 2010 at 09:48:08AM +0900, KOSAKI Motohiro wrote: > > > > + /* > > > > + * 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; > > > > + } > > > > > > I haven't understand what prevent this check. Why don't we need following scenario? > > > > > > 1. Page isolated for migration > > > 2. Passed this if (!page_mapcount(page)) check > > > 3. Process exits > > > 4. page_mapcount(page) drops to zero so anon_vma was no longer reliable > > > > > > Traditionally, page migration logic is, it can touch garbarge of anon_vma, but > > > SLAB_DESTROY_BY_RCU prevent any disaster. Is this broken concept? > > > > The check is made within the RCU read lock. If the count is positive at > > that point but goes to zero due to a process exiting, the anon_vma will > > still be valid until rcu_read_unlock() is called. > > Thank you! > > then, this logic depend on SLAB_DESTROY_BY_RCU, not refcount. > So, I think we don't need your [1/11] patch. > > Am I missing something? > The refcount is still needed. The anon_vma might be valid, but the refcount is what ensures that the anon_vma is not freed and reused. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id E77CB6B00D7 for ; Thu, 18 Mar 2010 07:14:56 -0400 (EDT) Date: Thu, 18 Mar 2010 11:14:36 +0000 From: Mel Gorman Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Message-ID: <20100318111436.GK12388@csn.ul.ie> References: <20100317104734.4C8E.A69D9226@jp.fujitsu.com> <20100317115133.GG12388@csn.ul.ie> <20100318094720.872F.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20100318094720.872F.A69D9226@jp.fujitsu.com> Sender: owner-linux-mm@kvack.org To: KOSAKI Motohiro Cc: Minchan Kim , KAMEZAWA Hiroyuki , Andrew Morton , Andrea Arcangeli , Christoph Lameter , Adam Litke , Avi Kivity , David Rientjes , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: On Thu, Mar 18, 2010 at 09:48:08AM +0900, KOSAKI Motohiro wrote: > > > > + /* > > > > + * 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; > > > > + } > > > > > > I haven't understand what prevent this check. Why don't we need following scenario? > > > > > > 1. Page isolated for migration > > > 2. Passed this if (!page_mapcount(page)) check > > > 3. Process exits > > > 4. page_mapcount(page) drops to zero so anon_vma was no longer reliable > > > > > > Traditionally, page migration logic is, it can touch garbarge of anon_vma, but > > > SLAB_DESTROY_BY_RCU prevent any disaster. Is this broken concept? > > > > The check is made within the RCU read lock. If the count is positive at > > that point but goes to zero due to a process exiting, the anon_vma will > > still be valid until rcu_read_unlock() is called. > > Thank you! > > then, this logic depend on SLAB_DESTROY_BY_RCU, not refcount. > So, I think we don't need your [1/11] patch. > > Am I missing something? > The refcount is still needed. The anon_vma might be valid, but the refcount is what ensures that the anon_vma is not freed and reused. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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