From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754376Ab0CRAsM (ORCPT ); Wed, 17 Mar 2010 20:48:12 -0400 Received: from fgwmail6.fujitsu.co.jp ([192.51.44.36]:37871 "EHLO fgwmail6.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753242Ab0CRAsL (ORCPT ); Wed, 17 Mar 2010 20:48:11 -0400 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 From: KOSAKI Motohiro To: Mel Gorman Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Cc: kosaki.motohiro@jp.fujitsu.com, 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 In-Reply-To: <20100317115133.GG12388@csn.ul.ie> References: <20100317104734.4C8E.A69D9226@jp.fujitsu.com> <20100317115133.GG12388@csn.ul.ie> Message-Id: <20100318094720.872F.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Mailer: Becky! ver. 2.50.07 [ja] Date: Thu, 18 Mar 2010 09:48:08 +0900 (JST) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > > > + /* > > > + * 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? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with SMTP id AB8E86B00DC for ; Wed, 17 Mar 2010 20:48:13 -0400 (EDT) Received: from m2.gw.fujitsu.co.jp ([10.0.50.72]) by fgwmail7.fujitsu.co.jp (Fujitsu Gateway) with ESMTP id o2I0mAXf018355 for (envelope-from kosaki.motohiro@jp.fujitsu.com); Thu, 18 Mar 2010 09:48:10 +0900 Received: from smail (m2 [127.0.0.1]) by outgoing.m2.gw.fujitsu.co.jp (Postfix) with ESMTP id 20AFB45DE55 for ; Thu, 18 Mar 2010 09:48:10 +0900 (JST) Received: from s2.gw.fujitsu.co.jp (s2.gw.fujitsu.co.jp [10.0.50.92]) by m2.gw.fujitsu.co.jp (Postfix) with ESMTP id E61F545DE4F for ; Thu, 18 Mar 2010 09:48:09 +0900 (JST) Received: from s2.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s2.gw.fujitsu.co.jp (Postfix) with ESMTP id CF0F11DB8038 for ; Thu, 18 Mar 2010 09:48:09 +0900 (JST) Received: from m108.s.css.fujitsu.com (m108.s.css.fujitsu.com [10.249.87.108]) by s2.gw.fujitsu.co.jp (Postfix) with ESMTP id 770AC1DB803B for ; Thu, 18 Mar 2010 09:48:09 +0900 (JST) From: KOSAKI Motohiro Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages In-Reply-To: <20100317115133.GG12388@csn.ul.ie> References: <20100317104734.4C8E.A69D9226@jp.fujitsu.com> <20100317115133.GG12388@csn.ul.ie> Message-Id: <20100318094720.872F.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Date: Thu, 18 Mar 2010 09:48:08 +0900 (JST) Sender: owner-linux-mm@kvack.org To: Mel Gorman Cc: kosaki.motohiro@jp.fujitsu.com, 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: > > > + /* > > > + * 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? -- 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