From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f198.google.com (mail-io0-f198.google.com [209.85.223.198]) by kanga.kvack.org (Postfix) with ESMTP id 64D6C6B0005 for ; Wed, 3 Aug 2016 07:46:48 -0400 (EDT) Received: by mail-io0-f198.google.com with SMTP id i199so439508294ioi.2 for ; Wed, 03 Aug 2016 04:46:48 -0700 (PDT) Received: from EUR03-VE1-obe.outbound.protection.outlook.com (mail-eopbgr50120.outbound.protection.outlook.com. [40.107.5.120]) by mx.google.com with ESMTPS id p125si4257487oih.232.2016.08.03.04.46.47 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Wed, 03 Aug 2016 04:46:47 -0700 (PDT) Date: Wed, 3 Aug 2016 14:46:40 +0300 From: Vladimir Davydov Subject: Re: [PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout from offline cgroup Message-ID: <20160803114639.GI13263@esperanza> References: <20160802160025.GB28900@dhcp22.suse.cz> <20160803095049.GG13263@esperanza> <20160803110941.GA19196@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20160803110941.GA19196@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Andrew Morton , stable@vger.kernel.org, Johannes Weiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Wed, Aug 03, 2016 at 01:09:42PM +0200, Michal Hocko wrote: > 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. But we can handle this IMO. AFAICS diverging id refcount will typically result in leaking swap charges, which aren't even a real resource. At worst, we can leak an offline mem_cgroup, which is also not critical enough to crash the production system. I see your concern of papering over a bug though. What about adding a warning there? diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 1c0aa59fd333..8c8e68becee9 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4044,7 +4044,7 @@ static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg) * The root cgroup cannot be destroyed, so it's refcount must * always be >= 1. */ - if (memcg == root_mem_cgroup) { + if (WARN_ON_ONCE(memcg == root_mem_cgroup)) { VM_BUG_ON(1); break; } -- 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