From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f71.google.com (mail-pl0-f71.google.com [209.85.160.71]) by kanga.kvack.org (Postfix) with ESMTP id 811766B000E for ; Thu, 26 Jul 2018 04:27:29 -0400 (EDT) Received: by mail-pl0-f71.google.com with SMTP id w18-v6so789440plp.3 for ; Thu, 26 Jul 2018 01:27:29 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id o23-v6si683749pgv.518.2018.07.26.01.27.28 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 26 Jul 2018 01:27:28 -0700 (PDT) Date: Thu, 26 Jul 2018 10:27:23 +0200 From: Michal Hocko Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps Message-ID: <20180726082723.GB28386@dhcp22.suse.cz> References: <20180723123043.GD31229@dhcp22.suse.cz> <8daae80c-871e-49b6-1cf1-1f0886d3935d@redhat.com> <20180724072536.GB28386@dhcp22.suse.cz> <8eb22489-fa6b-9825-bc63-07867a40d59b@redhat.com> <20180724131343.GK28386@dhcp22.suse.cz> <20180724133530.GN28386@dhcp22.suse.cz> <6c753cae-f8b6-5563-e5ba-7c1fefdeb74e@redhat.com> <20180725135147.GN28386@dhcp22.suse.cz> <344d5f15-c621-9973-561e-6ed96b29ea88@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <344d5f15-c621-9973-561e-6ed96b29ea88@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand Cc: Vlastimil Babka , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Baoquan He , Dave Young , Greg Kroah-Hartman , Hari Bathini , Huang Ying , "Kirill A. Shutemov" , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , Matthew Wilcox , Miles Chen , Pavel Tatashin , Petr Tesarik On Wed 25-07-18 16:20:41, David Hildenbrand wrote: > On 25.07.2018 15:51, Michal Hocko wrote: > > On Tue 24-07-18 16:13:09, David Hildenbrand wrote: > > [...] > >> So I see right now: > >> > >> - Pg_reserved + e.g. new page type (or some other unique identifier in > >> combination with Pg_reserved) > >> -> Avoid reads of pages we know are offline > >> - extend is_ram_page() > >> -> Fake zero memory for pages we know are offline > >> > >> Or even both (avoid reading and don't crash the kernel if it is being done). > > > > I really fail to see how that can work without kernel being aware of > > PageOffline. What will/should happen if you run an old kdump tool on a > > kernel with this partially offline memory? > > > > New kernel with old dump tool: > > a) we have not fixed up is_ram_page() > > -> crash, as we access memory we shouldn't this is not acceptable, right? You do not want to crash your crash kernel ;) > b) we have fixed up is_ram_page() > > -> We have a callback to check for applicable memory in the hypervisor > whether the parts are accessible / online or not accessible / offline. > (e.g. via a device driver that controls a certain memory region) > > -> Don't read, but fake a page full of 0 > > > So instead of the kernel being aware of it, it asks via is_ram_page() > the hypervisor. I am still confused why do we even care about hypervisor. What if somebody wants to have partial memory hotplug on native OS? > I don't think a) is a problem. AFAICS, we have to update makedumpfile > for every new kernel. We can perform changes and update makedumpfile > to be compatible with new dump tools. Not really. You simply do not crash the kernel just because you are trying to dump the already crashed kernel. > E.g. remember SECTION_IS_ONLINE you introduced ? It broke dump > tools and required But has it crashed the kernel when reading the dump? If yes then the whole dumping is fragile as hell... -- Michal Hocko SUSE Labs