From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 85F7E6B0007 for ; Tue, 24 Jul 2018 03:25:39 -0400 (EDT) Received: by mail-ed1-f72.google.com with SMTP id o60-v6so1352559edd.13 for ; Tue, 24 Jul 2018 00:25:39 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id q8-v6si9625452edn.6.2018.07.24.00.25.38 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 24 Jul 2018 00:25:38 -0700 (PDT) Date: Tue, 24 Jul 2018 09:25:36 +0200 From: Michal Hocko Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps Message-ID: <20180724072536.GB28386@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8daae80c-871e-49b6-1cf1-1f0886d3935d@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 Mon 23-07-18 19:20:43, David Hildenbrand wrote: > On 23.07.2018 14:30, Michal Hocko wrote: > > On Mon 23-07-18 13:45:18, Vlastimil Babka wrote: > >> On 07/20/2018 02:34 PM, David Hildenbrand wrote: > >>> Dumping tools (like makedumpfile) right now don't exclude reserved pages. > >>> So reserved pages might be access by dump tools although nobody except > >>> the owner should touch them. > >> > >> Are you sure about that? Or maybe I understand wrong. Maybe it changed > >> recently, but IIRC pages that are backing memmap (struct pages) are also > >> PG_reserved. And you definitely do want those in the dump. > > > > You are right. reserve_bootmem_region will make all early bootmem > > allocations (including those backing memmaps) PageReserved. I have asked > > several times but I haven't seen a satisfactory answer yet. Why do we > > even care for kdump about those. If they are reserved the nobody should > > really look at those specific struct pages and manipulate them. Kdump > > tools are using a kernel interface to read the content. If the specific > > content is backed by a non-existing memory then they should simply not > > return anything. > > > > "new kernel" provides an interface to read memory from "old kernel". > > The new kernel has no idea about > - which memory was added/online in the old kernel > - where struct pages of the old kernel are and what their content is > - which memory is save to touch and which not > > Dump tools figure all that out by interpreting the VMCORE. They e.g. > identify "struct pages" and see if they should be dumped. The "new > kernel" only allows to read that memory. It cannot hinder to crash the > system (e.g. if a dump tool would try to read a hwpoison page). > > So how should the "new kernel" know if a page can be touched or not? I am sorry I am not familiar with kdump much. But from what I remember it reads from /proc/vmcore and implementation of this interface should simply return EINVAL or alike when you try to dump inaccessible memory range. > The *only* way would be to have an interface to the hypervisor where we > "sense" if a memory location is safe to touch. I remember that xen or > hyper-v does that - they fake a zero page in that case, after querying > the hypervisor. But this does not sound like a clean approach to me, > especially es we need yet another hypervisor interface to sense for > memory provided via "some" device. > > If we can find a way to just tag pages as "don't touch", it would be the > easiest and cleanest solution in my opinion. If only we could have much more spare room in struct pages... -- Michal Hocko SUSE Labs