From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753205Ab0CYJ7d (ORCPT ); Thu, 25 Mar 2010 05:59:33 -0400 Received: from fgwmail6.fujitsu.co.jp ([192.51.44.36]:37536 "EHLO fgwmail6.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752506Ab0CYJ7b (ORCPT ); Thu, 25 Mar 2010 05:59:31 -0400 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 From: KOSAKI Motohiro To: KAMEZAWA Hiroyuki Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Cc: kosaki.motohiro@jp.fujitsu.com, Mel Gorman , 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 In-Reply-To: <20100325184123.e3e3b009.kamezawa.hiroyu@jp.fujitsu.com> References: <20100325092131.GK2024@csn.ul.ie> <20100325184123.e3e3b009.kamezawa.hiroyu@jp.fujitsu.com> Message-Id: <20100325185200.6C8C.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, 25 Mar 2010 18:59:25 +0900 (JST) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > > > > 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. thanks. > and > == > void anon_vma_free(struct anon_vma *anon) > { > /* > * To increase refcnt of anon-vma, anon_vma->lock should be held by > * page_lock_anon_vma(). It means anon_vma has a "mapped" page. > * If this anon is freed by unmap or exit, all pages under this anon > * must be unmapped. Then, just checking refcnt without lock is ok. > */ > if (check refcnt > 0) > return do nothing > kmem_cache_free(anon); > } > == > > Then, rcu_read_lock can be removed in clean way. > > Thanks, > -Kame > > > -- > 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