From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f199.google.com (mail-qk0-f199.google.com [209.85.220.199]) by kanga.kvack.org (Postfix) with ESMTP id AB1046B02BE for ; Wed, 25 Jul 2018 10:20:47 -0400 (EDT) Received: by mail-qk0-f199.google.com with SMTP id z18-v6so6522208qki.22 for ; Wed, 25 Jul 2018 07:20:47 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id e2-v6si1537389qki.380.2018.07.25.07.20.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 25 Jul 2018 07:20:46 -0700 (PDT) Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps References: <20180720123422.10127-1-david@redhat.com> <9f46f0ed-e34c-73be-60ca-c892fb19ed08@suse.cz> <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> From: David Hildenbrand Message-ID: <344d5f15-c621-9973-561e-6ed96b29ea88@redhat.com> Date: Wed, 25 Jul 2018 16:20:41 +0200 MIME-Version: 1.0 In-Reply-To: <20180725135147.GN28386@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 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 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 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. E.g. remember SECTION_IS_ONLINE you introduced ? It broke dump tools and required commit 4bf4f2b0a855ccf4c7ffe13290778e92b2f5bbc9 Author: Pratyush Anand Date: Thu Aug 17 12:47:13 2017 +0900 [PATCH v2] Fix SECTION_MAP_MASK for kernel >= v.13 * Required for kernel 4.13 commit 2d070eab2e82 "mm: consider zone which is not fully populated to have holes" added a new flag SECTION_IS_ONLINE and therefore SECTION_MAP_MASK has been changed. We are not able to find correct mem_map in makedumpfile for kernel version v4.13-rc1 and onward because of the above kernel change. This patch fixes the MASK value keeping the code backward compatible Signed-off-by: Pratyush Anand Same would apply for the new combination of PageReserved + X, where we tell dump tools to exclude this page. -- Thanks, David / dhildenb