From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id DD51D6B0071 for ; Thu, 25 Mar 2010 05:45:12 -0400 (EDT) Received: from m4.gw.fujitsu.co.jp ([10.0.50.74]) by fgwmail7.fujitsu.co.jp (Fujitsu Gateway) with ESMTP id o2P9j923022020 for (envelope-from kamezawa.hiroyu@jp.fujitsu.com); Thu, 25 Mar 2010 18:45:10 +0900 Received: from smail (m4 [127.0.0.1]) by outgoing.m4.gw.fujitsu.co.jp (Postfix) with ESMTP id 6275045DE7D for ; Thu, 25 Mar 2010 18:45:09 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (s4.gw.fujitsu.co.jp [10.0.50.94]) by m4.gw.fujitsu.co.jp (Postfix) with ESMTP id 343D445DE7A for ; Thu, 25 Mar 2010 18:45:09 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id 0AD01E1800D for ; Thu, 25 Mar 2010 18:45:09 +0900 (JST) Received: from ml14.s.css.fujitsu.com (ml14.s.css.fujitsu.com [10.249.87.104]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id 98D73E18006 for ; Thu, 25 Mar 2010 18:45:08 +0900 (JST) Date: Thu, 25 Mar 2010 18:41:23 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Message-Id: <20100325184123.e3e3b009.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20100325092131.GK2024@csn.ul.ie> References: <20100325083235.GF2024@csn.ul.ie> <20100325180221.e1d9bae7.kamezawa.hiroyu@jp.fujitsu.com> <20100325180726.6C89.A69D9226@jp.fujitsu.com> <20100325092131.GK2024@csn.ul.ie> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Mel Gorman 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 List-ID: On Thu, 25 Mar 2010 09:21:32 +0000 Mel Gorman wrote: > On Thu, Mar 25, 2010 at 06:09:34PM +0900, KOSAKI Motohiro wrote: > > > On Thu, 25 Mar 2010 08:32:35 +0000 > > > Mel Gorman wrote: > > > > > > > On Thu, Mar 25, 2010 at 11:49:23AM +0900, KOSAKI Motohiro wrote: > > > > > > On Fri, Mar 19, 2010 at 03:21:41PM +0900, KOSAKI Motohiro wrote: > > > > > Hmmm... > > > > > I haven't understand your mention because I guess I was wrong. > > > > > > > > > > probably my last question was unclear. I mean, > > > > > > > > > > 1) If we still need SLAB_DESTROY_BY_RCU, why do we need to add refcount? > > > > > Which difference is exist between normal page migration and compaction? > > > > > > > > The processes typically calling migration today own the page they are moving > > > > and is not going to exit unexpectedly during migration. > > > > > > > > > 2) If we added refcount, which race will solve? > > > > > > > > > > > > > The process exiting and the last anon_vma being dropped while compaction > > > > is running. This can be reliably triggered with compaction. > > > > > > > > > IOW, Is this patch fix old issue or compaction specific issue? > > > > > > > > > > > > > Strictly speaking, it's an old issue but in practice it's impossible to > > > > trigger because the process migrating always owns the page. Compaction > > > > moves pages belonging to arbitrary processes. > > > > > > > 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); } .... == 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