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 DC9606B006A for ; Fri, 26 Jun 2009 08:48:20 -0400 (EDT) Received: from d01relay02.pok.ibm.com (d01relay02.pok.ibm.com [9.56.227.234]) by e3.ny.us.ibm.com (8.13.1/8.13.1) with ESMTP id n5QCiXM1013704 for ; Fri, 26 Jun 2009 08:44:33 -0400 Received: from d01av04.pok.ibm.com (d01av04.pok.ibm.com [9.56.224.64]) by d01relay02.pok.ibm.com (8.13.8/8.13.8/NCO v9.2) with ESMTP id n5QCnlWX247100 for ; Fri, 26 Jun 2009 08:49:47 -0400 Received: from d01av04.pok.ibm.com (loopback [127.0.0.1]) by d01av04.pok.ibm.com (8.12.11.20060308/8.13.3) with ESMTP id n5QCnjnu016683 for ; Fri, 26 Jun 2009 08:49:46 -0400 Date: Fri, 26 Jun 2009 10:18:03 +0530 From: Balbir Singh Subject: Re: [PATCH] memcg: add commens for expaing memory barrier (Was Re: Low overhead patches for the memory cgroup controller (v5) Message-ID: <20090626044803.GG8642@balbir.in.ibm.com> Reply-To: balbir@linux.vnet.ibm.com References: <20090615043900.GF23577@balbir.in.ibm.com> <20090622154343.9cdbf23a.akpm@linux-foundation.org> <20090623090116.556d4f97.kamezawa.hiroyu@jp.fujitsu.com> <20090626095745.01cef410.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20090626095745.01cef410.kamezawa.hiroyu@jp.fujitsu.com> Sender: owner-linux-mm@kvack.org To: KAMEZAWA Hiroyuki Cc: Andrew Morton , kamezawa.hiroyuki@jp.fujitsu.com, nishimura@mxp.nes.nec.co.jp, lizf@cn.fujitsu.com, menage@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: * KAMEZAWA Hiroyuki [2009-06-26 09:57:45]: > On Tue, 23 Jun 2009 09:01:16 +0900 > KAMEZAWA Hiroyuki wrote: > > > Do we still need the smp_wmb()? > > > > > > It's hard to say, because we forgot to document it :( > > > > > Sorry for lack of documentation. > > > > pc->mem_cgroup should be visible before SetPageCgroupUsed(). Othrewise, > > A routine believes USED bit will see bad pc->mem_cgroup. > > > > I'd like to add a comment later (againt new mmotm.) > > > > Ok, it's now. > == > From: KAMEZAWA Hiroyuki > > Add comments for the reason of smp_wmb() in mem_cgroup_commit_charge(). > > Cc: Daisuke Nishimura > Cc: Balbir Singh > Signed-off-by: KAMEZAWA Hiroyuki > --- > mm/memcontrol.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > Index: mmotm-2.6.31-Jun25/mm/memcontrol.c > =================================================================== > --- mmotm-2.6.31-Jun25.orig/mm/memcontrol.c > +++ mmotm-2.6.31-Jun25/mm/memcontrol.c > @@ -1134,6 +1134,13 @@ static void __mem_cgroup_commit_charge(s > } > > pc->mem_cgroup = mem; > + /* > + * We access a page_cgroup asynchronously without lock_page_cgroup(). > + * Especially when a page_cgroup is taken from a page, pc->mem_cgroup > + * is accessed after testing USED bit. To make pc->mem_cgroup visible > + * before USED bit, we need memory barrier here. > + * See mem_cgroup_add_lru_list(), etc. > + */ I don't think this is sufficient, since in mem_cgroup_get_reclaim_stat_from_page() we say we need this since we set used bit without atomic operation. The used bit is now atomically set. I think we need to reword other comments as well. -- Balbir -- 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