From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753503Ab0CYNkB (ORCPT ); Thu, 25 Mar 2010 09:40:01 -0400 Received: from gir.skynet.ie ([193.1.99.77]:35313 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751283Ab0CYNkA (ORCPT ); Thu, 25 Mar 2010 09:40:00 -0400 Date: Thu, 25 Mar 2010 13:39:36 +0000 From: Mel Gorman To: KAMEZAWA Hiroyuki Cc: KOSAKI Motohiro , Minchan Kim , 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: <20100325133936.GR2024@csn.ul.ie> References: <20100325092131.GK2024@csn.ul.ie> <20100325184123.e3e3b009.kamezawa.hiroyu@jp.fujitsu.com> <20100325185200.6C8C.A69D9226@jp.fujitsu.com> <20100325191229.8e3d2ba1.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20100325191229.8e3d2ba1.kamezawa.hiroyu@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 25, 2010 at 07:12:29PM +0900, KAMEZAWA Hiroyuki wrote: > On Thu, 25 Mar 2010 18:59:25 +0900 (JST) > KOSAKI Motohiro wrote: > > > > > > > Kosaki-san, > > > > > > > > > > > > IIUC, the race in memory-hotunplug was fixed by this patch [2/11]. > > > > > > > > > > > > But, this behavior of unmap_and_move() requires access to _freed_ > > > > > > objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU, > > > > > > it't not good habit in general. > > > > > > > > > > > > After direct compaction, page-migration will be one of "core" code of > > > > > > memory management. Then, I agree to patch [1/11] as our direction for > > > > > > keeping sanity and showing direction to more updates. Maybe adding > > > > > > refcnt and removing RCU in futuer is good. > > > > > > > > > > But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount > > > > > is meaningless now. > > > > > > > > Christoph is opposed to removing it because of cache-hotness issues more > > > > so than use-after-free concerns. The refcount is needed with or without > > > > SLAB_DESTROY_BY_RCU. > > > > > > > > > > I wonder a code which the easiest to be read will be like following. > > > == > > > > > > if (PageAnon(page)) { > > > struct anon_vma anon = page_lock_anon_vma(page); > > > /* to take this lock, this page must be mapped. */ > > > if (!anon_vma) > > > goto uncharge; > > > increase refcnt > > > page_unlock_anon_vma(anon); > > > } > > > .... > > > == > > > > This seems very good and acceptable to me. This refcnt usage > > obviously reduce rcu-lock holding time. > > > > I still think no refcount doesn't cause any disaster. but I agree > > this is forward step patch. > > > > BTW, by above change and the change in patch [2/11], > "A page turnd to be SwapCache and free unmapped but not freed" > page will be never migrated. > Good point. > Mel, could you change the check as this ?? > > if (PageAnon(page)) { > rcu_read_lock(); > if (!page_mapcount(page)) { > rcu_read_unlock(); > if (!PageSwapCache(page)) > goto uncharge; > /* unmapped swap cache can be migrated */ > } else { > ... > } > ..... > } else > There were minor changes in how the rcu_read_lock is taken and released based on other comments. With your suggestion, the block now looks like; if (PageAnon(page)) { rcu_read_lock(); rcu_locked = 1; /* * 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) && !PageSwapCache(page)) goto rcu_unlock; anon_vma = page_anon_vma(page); atomic_inc(&anon_vma->external_refcount); } Thanks. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab