From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with SMTP id 19B186B0071 for ; Thu, 25 Mar 2010 06:17:00 -0400 (EDT) Received: from m1.gw.fujitsu.co.jp ([10.0.50.71]) by fgwmail5.fujitsu.co.jp (Fujitsu Gateway) with ESMTP id o2PAGGE1026756 for (envelope-from kamezawa.hiroyu@jp.fujitsu.com); Thu, 25 Mar 2010 19:16:16 +0900 Received: from smail (m1 [127.0.0.1]) by outgoing.m1.gw.fujitsu.co.jp (Postfix) with ESMTP id 5212D45DE4E for ; Thu, 25 Mar 2010 19:16:16 +0900 (JST) Received: from s1.gw.fujitsu.co.jp (s1.gw.fujitsu.co.jp [10.0.50.91]) by m1.gw.fujitsu.co.jp (Postfix) with ESMTP id 1ABD145DE4F for ; Thu, 25 Mar 2010 19:16:16 +0900 (JST) Received: from s1.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s1.gw.fujitsu.co.jp (Postfix) with ESMTP id D8D33E38002 for ; Thu, 25 Mar 2010 19:16:15 +0900 (JST) Received: from m105.s.css.fujitsu.com (m105.s.css.fujitsu.com [10.249.87.105]) by s1.gw.fujitsu.co.jp (Postfix) with ESMTP id 89CA91DB804A for ; Thu, 25 Mar 2010 19:16:15 +0900 (JST) Date: Thu, 25 Mar 2010 19:12:29 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Message-Id: <20100325191229.8e3d2ba1.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20100325185200.6C8C.A69D9226@jp.fujitsu.com> References: <20100325092131.GK2024@csn.ul.ie> <20100325184123.e3e3b009.kamezawa.hiroyu@jp.fujitsu.com> <20100325185200.6C8C.A69D9226@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: KOSAKI Motohiro Cc: 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 List-ID: 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. 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 Thx, -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