From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id 176A1900138 for ; Thu, 8 Sep 2011 20:29:40 -0400 (EDT) Received: from m1.gw.fujitsu.co.jp (unknown [10.0.50.71]) by fgwmail5.fujitsu.co.jp (Postfix) with ESMTP id 380783EE0BD for ; Fri, 9 Sep 2011 09:29:36 +0900 (JST) Received: from smail (m1 [127.0.0.1]) by outgoing.m1.gw.fujitsu.co.jp (Postfix) with ESMTP id 1B08645DE56 for ; Fri, 9 Sep 2011 09:29:36 +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 EC14145DE53 for ; Fri, 9 Sep 2011 09:29:35 +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 DEEE91DB8050 for ; Fri, 9 Sep 2011 09:29:35 +0900 (JST) Received: from m105.s.css.fujitsu.com (m105.s.css.fujitsu.com [10.240.81.145]) by s1.gw.fujitsu.co.jp (Postfix) with ESMTP id 9B48B1DB8032 for ; Fri, 9 Sep 2011 09:29:35 +0900 (JST) Date: Fri, 9 Sep 2011 09:28:53 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [patch] mm: memcg: close race between charge and putback Message-Id: <20110909092853.7aac8544.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20110908095349.GC1316@redhat.com> References: <1315467622-9520-1-git-send-email-jweiner@redhat.com> <20110908173042.4a6f8ac0.kamezawa.hiroyu@jp.fujitsu.com> <20110908085404.GA1316@redhat.com> <20110908181901.1d488d73.kamezawa.hiroyu@jp.fujitsu.com> <20110908093316.GB1316@redhat.com> <20110908184221.feb2dab6.kamezawa.hiroyu@jp.fujitsu.com> <20110908095349.GC1316@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Andrew Morton , Ying Han , Daisuke Nishimura , Balbir Singh , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Thu, 8 Sep 2011 11:53:49 +0200 Johannes Weiner wrote: > On Thu, Sep 08, 2011 at 06:42:21PM +0900, KAMEZAWA Hiroyuki wrote: > > On Thu, 8 Sep 2011 11:33:16 +0200 > > Johannes Weiner wrote: > > > > > On Thu, Sep 08, 2011 at 06:19:01PM +0900, KAMEZAWA Hiroyuki wrote: > > > > On Thu, 8 Sep 2011 10:54:04 +0200 > > > > Johannes Weiner wrote: > > > > > > > > > On Thu, Sep 08, 2011 at 05:30:42PM +0900, KAMEZAWA Hiroyuki wrote: > > > > > > On Thu, 8 Sep 2011 09:40:22 +0200 > > > > > > Johannes Weiner wrote: > > > > > > > > > > > > > There is a potential race between a thread charging a page and another > > > > > > > thread putting it back to the LRU list: > > > > > > > > > > > > > > charge: putback: > > > > > > > SetPageCgroupUsed SetPageLRU > > > > > > > PageLRU && add to memcg LRU PageCgroupUsed && add to memcg LRU > > > > > > > > > > > > > > > > > > > I assumed that all pages are charged before added to LRU. > > > > > > (i.e. event happens in charge->lru_lock->putback order.) > > > > > > > > > > > > But hmm, this assumption may be bad for maintainance. > > > > > > Do you find a code which adds pages to LRU before charge ? > > > > > > > > > > > > Hmm, if there are codes which recharge the page to other memcg, > > > > > > it will cause bug and my assumption may be harmful. > > > > > > > > > > Swap slots are read optimistically into swapcache and put to the LRU, > > > > > then charged upon fault. > > > > > > > > Yes, then swap charge removes page from LRU before charge. > > > > IIUC, it needed to do so because page->mem_cgroup may be replaced. > > > > > > But only from the memcg LRU. It's still on the global per-zone LRU, > > > so reclaim could isolate/putback it during the charge. And then > > > > > > > > > > charge: putback: > > > > > > > SetPageCgroupUsed SetPageLRU > > > > > > > PageLRU && add to memcg LRU PageCgroupUsed && add to memcg LRU > > > > > > applies. > > > > Hmm, in this case, I thought memcg puts back the page to its LRU by itself > > under lru_loc after charge and the race was hidden. > > But it locklessly checks PageLRU and bails if it's cleared and that is I think PageLRU check is done under zone->lru_lock. > the problem: it's not guaranteed that PageLRU is observed on the > charging CPU when the putback side bailed because of PageCgroupUsed. > zone->lru_lock is no help ? > My barrier puts this in order and makes sure one of the two succeeds. > 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org