From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751790AbdBTRnS (ORCPT ); Mon, 20 Feb 2017 12:43:18 -0500 Received: from mx2.suse.de ([195.135.220.15]:42567 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750920AbdBTRnQ (ORCPT ); Mon, 20 Feb 2017 12:43:16 -0500 Date: Mon, 20 Feb 2017 18:42:59 +0100 From: Michal Hocko To: Laurent Dufour Cc: Johannes Weiner , Vladimir Davydov , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/cgroup: avoid panic when init with low memory Message-ID: <20170220174258.GA31541@dhcp22.suse.cz> References: <1487154969-6704-1-git-send-email-ldufour@linux.vnet.ibm.com> <20170220130123.GI2431@dhcp22.suse.cz> <934d40ec-060b-4794-2fdc-35a7ea1dc9e2@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <934d40ec-060b-4794-2fdc-35a7ea1dc9e2@linux.vnet.ibm.com> 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 Mon 20-02-17 18:09:43, Laurent Dufour wrote: > On 20/02/2017 14:01, Michal Hocko wrote: > > On Wed 15-02-17 11:36:09, Laurent Dufour wrote: > >> The system may panic when initialisation is done when almost all the > >> memory is assigned to the huge pages using the kernel command line > >> parameter hugepage=xxxx. Panic may occur like this: > > > > I am pretty sure the system might blow up in many other ways when you > > misconfigure it and pull basically all the memory out. Anyway... > > > > [...] > > > >> This is a chicken and egg issue where the kernel try to get free > >> memory when allocating per node data in mem_cgroup_init(), but in that > >> path mem_cgroup_soft_limit_reclaim() is called which assumes that > >> these data are allocated. > >> > >> As mem_cgroup_soft_limit_reclaim() is best effort, it should return > >> when these data are not yet allocated. > > > > ... this makes some sense. Especially when there is no soft limit > > configured. So this is a good step. I would just like to ask you to go > > one step further. Can we make the whole soft reclaim thing uninitialized > > until the soft limit is actually set? Soft limit is not used in cgroup > > v2 at all and I would strongly discourage it in v1 as well. We will save > > few bytes as a bonus. > > Hi Michal, and thanks for the review. > > I'm not familiar with that part of the kernel, so to be sure we are on > the same line, are you suggesting to set soft_limit_tree at the first > time mem_cgroup_write() is called to set a soft_limit field ? yes > Obviously, all callers to soft_limit_tree_node() and > soft_limit_tree_from_page() will have to check for the return pointer to > be NULL. All callers that need to access the tree unconditionally, yes. Which is the case anyway, right? I haven't checked the check you have added is sufficient, but we shouldn't have that many of them because some code paths are called only when the soft limit is enabled. -- Michal Hocko SUSE Labs