From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f197.google.com (mail-qt0-f197.google.com [209.85.216.197]) by kanga.kvack.org (Postfix) with ESMTP id 012526B0278 for ; Thu, 26 Jul 2018 04:37:42 -0400 (EDT) Received: by mail-qt0-f197.google.com with SMTP id o18-v6so673676qtm.11 for ; Thu, 26 Jul 2018 01:37:41 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id w44-v6si715543qtg.179.2018.07.26.01.37.41 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 26 Jul 2018 01:37:41 -0700 (PDT) Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps 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> <20180726082723.GB28386@dhcp22.suse.cz> From: David Hildenbrand Message-ID: <60975612-9b91-65dd-03d8-579ba23a6c01@redhat.com> Date: Thu, 26 Jul 2018 10:37:33 +0200 MIME-Version: 1.0 In-Reply-To: <20180726082723.GB28386@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko 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" , =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= , Matthew Wilcox , Miles Chen , Pavel Tatashin , Petr Tesarik On 26.07.2018 10:27, Michal Hocko wrote: > 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 ;) Well, the same can happen today with PageHWPoison. The "new" kernel will happily access such pages and crash as far as I understand (it has has no idea of the old struct pages). Of course, this is "less likely" than what I describe. > >> 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? Good point I was ignoring so far (too much focusing on my use case I assume). So for these, we would have to catch illegal accesses and a) report them (-EINVAL / - EIO) as you said b) fake a zero page I assume catching illegal accesses should be possible. Might require some work across all architectures. Still, dump tools should in addition not even try to read if possible. > >> 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... No, I think it simply didn't work. At least that's what I assume ;) I was rather saying that dump tools may have to be fixed up to work with a new kernel. -- Thanks, David / dhildenb