From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f52.google.com (mail-pa0-f52.google.com [209.85.220.52]) by kanga.kvack.org (Postfix) with ESMTP id 6B6AA6B0032 for ; Tue, 24 Feb 2015 20:20:10 -0500 (EST) Received: by padbj1 with SMTP id bj1so1011815pad.5 for ; Tue, 24 Feb 2015 17:20:10 -0800 (PST) Received: from lgeamrelo01.lge.com (lgeamrelo01.lge.com. [156.147.1.125]) by mx.google.com with ESMTP id tw8si19599120pbc.2.2015.02.24.17.20.08 for ; Tue, 24 Feb 2015 17:20:09 -0800 (PST) Date: Wed, 25 Feb 2015 10:20:13 +0900 From: Joonsoo Kim Subject: Re: [PATCH] mm/memcontrol: fix NULL pointer dereference when use_hierarchy is 0 Message-ID: <20150225012013.GC16796@js1304-P5Q-DELUXE> References: <1424150699-5395-1-git-send-email-iamjoonsoo.kim@lge.com> <20150217083327.GA32017@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150217083327.GA32017@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Andrew Morton , Johannes Weiner , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On Tue, Feb 17, 2015 at 09:33:27AM +0100, Michal Hocko wrote: > On Tue 17-02-15 14:24:59, Joonsoo Kim wrote: > > It can be possible to return NULL in parent_mem_cgroup() > > if use_hierarchy is 0. > > This alone is not sufficient because the low limit is present only in > the unified hierarchy API and there is no use_hierarchy there. The > primary issue here is that the memcg has 0 usage so the previous > check for usage will not stop us. And that is bug IMO. > > I think that the following patch would be more correct from semantic > POV: > --- > >From f5d74671d30e44c50b45b4464c92f536f1dbdff6 Mon Sep 17 00:00:00 2001 > From: Michal Hocko > Date: Tue, 17 Feb 2015 08:02:12 +0100 > Subject: [PATCH] memcg: fix low limit calculation > > A memcg is considered low limited even when the current usage is equal > to the low limit. This leads to interesting side effects e.g. > groups/hierarchies with no memory accounted are considered protected and > so the reclaim will emit MEMCG_LOW event when encountering them. > > Another and much bigger issue was reported by Joonsoo Kim. He has hit a > NULL ptr dereference with the legacy cgroup API which even doesn't have > low limit exposed. The limit is 0 by default but the initial check fails > for memcg with 0 consumption and parent_mem_cgroup() would return NULL > if use_hierarchy is 0 and so page_counter_read would try to dereference > NULL. > > I suppose that the current implementation is just an overlook because > the documentation in Documentation/cgroups/unified-hierarchy.txt says: > " > The memory.low boundary on the other hand is a top-down allocated > reserve. A cgroup enjoys reclaim protection when it and all its > ancestors are below their low boundaries > " > > Fix the usage and the low limit comparision in mem_cgroup_low accordingly. > > Fixes: 241994ed8649 (mm: memcontrol: default hierarchy interface for memory) > Reported-by: Joonsoo Kim > Signed-off-by: Michal Hocko Good! This fixes my issue. Thanks. -- 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