From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934184Ab1JEJX6 (ORCPT ); Wed, 5 Oct 2011 05:23:58 -0400 Received: from e28smtp06.in.ibm.com ([122.248.162.6]:52968 "EHLO e28smtp06.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934139Ab1JEJX5 (ORCPT ); Wed, 5 Oct 2011 05:23:57 -0400 Date: Wed, 5 Oct 2011 14:53:50 +0530 From: "K.Prasad" To: Borislav Petkov , Vivek Goyal , linux-kernel@vger.kernel.org, crash-utility@redhat.com, kexec@lists.infradead.org, Andi Kleen , "Luck, Tony" , "Eric W. Biederman" , anderson@redhat.com, tachibana@mxm.nes.nec.co.jp, oomichi@mxs.nes.nec.co.jp Subject: Re: [Patch 1/4][kernel][slimdump] Add new elf-note of type NT_NOCOREDUMP to capture slimdump Message-ID: <20111005092350.GA7485@in.ibm.com> Reply-To: prasad@linux.vnet.ibm.com References: <20111003070735.GJ2223@in.ibm.com> <20111003073203.GA22694@in.ibm.com> <20111004140437.GA28306@redhat.com> <20111005071844.GB2235@in.ibm.com> <20111005073313.GB13478@liondog.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111005073313.GB13478@liondog.tnic> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 05, 2011 at 09:33:13AM +0200, Borislav Petkov wrote: > On Wed, Oct 05, 2011 at 12:48:44PM +0530, K.Prasad wrote: > > On Tue, Oct 04, 2011 at 10:04:37AM -0400, Vivek Goyal wrote: > > > On Mon, Oct 03, 2011 at 01:02:03PM +0530, K.Prasad wrote: > > > > There are certain types of crashes induced by faulty hardware in which > > > > capturing crashing kernel's memory (through kdump) makes no sense (or sometimes > > > > dangerous). > > > > > > > > A case in point, is unrecoverable memory errors (resulting in fatal machine > > > > check exceptions) in which reading from the faulty memory location from the > > > > kexec'ed kernel will cause double fault and system reset (leaving no > > > > information for the user). > > > > > > Prasad, > > > > > > I am just trying to remember what was wrong with Andi's approach of > > > disable MCE while copying the dump? > > > > > > > Hi Vivek, > > The behaviour upon a read operation on an UC memory location is > > undefined and so we want to avoid it (previously discussed here: > > http://article.gmane.org/gmane.linux.kernel/1146799). When we disable > > MCE and copy the dump, we will invariably read the faulty memory > > location. > > Right, from the message above: > > "- To disable MCE exceptions as done by the patches cited above. However > the result of a read operation on corrupted memory is unknown and the > system behaviour is undefined. We're unsure if this is a safe thing to > do." > > Can you elaborate more on that? Are we talking poisoned memory here or > undetected and uncorrectable memory errors? > It refers to uncorrected memory errors that are not consumed and the corresponding 'struct page's are marked PG_hwpoison. Typically the SRAO type errors that are handled in mm/memory-failure.c. If MCE is enabled, during a kdump, we will deliberately trigger a read operation over the poisoned memory and make the UCE fatal. It is not clear what would happen if MCE is disabled in the above case. Thanks, K.Prasad