From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754570AbcHCL16 (ORCPT ); Wed, 3 Aug 2016 07:27:58 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:36836 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752270AbcHCL1z (ORCPT ); Wed, 3 Aug 2016 07:27:55 -0400 Date: Wed, 3 Aug 2016 13:09:42 +0200 From: Michal Hocko To: Vladimir Davydov Cc: Andrew Morton , stable@vger.kernel.org, Johannes Weiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout from offline cgroup Message-ID: <20160803110941.GA19196@dhcp22.suse.cz> References: <20160802160025.GB28900@dhcp22.suse.cz> <20160803095049.GG13263@esperanza> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160803095049.GG13263@esperanza> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 03-08-16 12:50:49, Vladimir Davydov wrote: > On Tue, Aug 02, 2016 at 06:00:26PM +0200, Michal Hocko wrote: > > On Tue 02-08-16 18:00:48, Vladimir Davydov wrote: > ... > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 3be791afd372..4ae12effe347 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -4036,6 +4036,24 @@ static void mem_cgroup_id_get(struct mem_cgroup *memcg) > > > atomic_inc(&memcg->id.ref); > > > } > > > > > > +static struct mem_cgroup *mem_cgroup_id_get_active(struct mem_cgroup *memcg) > > > +{ > > > + while (!atomic_inc_not_zero(&memcg->id.ref)) { > > > + /* > > > + * The root cgroup cannot be destroyed, so it's refcount must > > > + * always be >= 1. > > > + */ > > > + if (memcg == root_mem_cgroup) { > > > + VM_BUG_ON(1); > > > + break; > > > + } > > > > why not simply VM_BUG_ON(memcg == root_mem_cgroup)? > > Because with DEBUG_VM disabled we could wind up looping forever here if > the refcount of the root_mem_cgroup got screwed up. On production > kernels, it's better to break the loop and carry on closing eyes on > diverging counters rather than getting a lockup. Wouldn't this just paper over a real bug? Anyway I will not insist but making the code more complex just to pretend we can handle a situation gracefully doesn't sound right to me. -- Michal Hocko SUSE Labs