From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0AC2FC33CAC for ; Mon, 13 Jan 2020 09:49:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DF4AA2082E for ; Mon, 13 Jan 2020 09:49:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728709AbgAMJtB (ORCPT ); Mon, 13 Jan 2020 04:49:01 -0500 Received: from foss.arm.com ([217.140.110.172]:36528 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725992AbgAMJtA (ORCPT ); Mon, 13 Jan 2020 04:49:00 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id CAE42106F; Mon, 13 Jan 2020 01:48:59 -0800 (PST) Received: from [10.162.43.142] (p8cg001049571a15.blr.arm.com [10.162.43.142]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 81C9D3F534; Mon, 13 Jan 2020 01:48:52 -0800 (PST) Subject: Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range To: David Hildenbrand Cc: David Hildenbrand , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com, cai@lca.pw, logang@deltatee.com, cpandya@codeaurora.org, arunks@codeaurora.org, dan.j.williams@intel.com, mgorman@techsingularity.net, osalvador@suse.de, ard.biesheuvel@arm.com, steve.capper@arm.com, broonie@kernel.org, valentin.schneider@arm.com, robin.murphy@arm.com, steven.price@arm.com, suzuki.poulose@arm.com, ira.weiny@intel.com References: <6f0efddc-f124-58ca-28b6-4632469cf992@arm.com> <3C3BE5FA-0CFC-4C90-8657-63EF5B680B0B@redhat.com> From: Anshuman Khandual Message-ID: <6b8fb779-31e8-1b63-85a8-9f6c93a04494@arm.com> Date: Mon, 13 Jan 2020 15:20:09 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <3C3BE5FA-0CFC-4C90-8657-63EF5B680B0B@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/13/2020 02:44 PM, David Hildenbrand wrote: > > >> Am 13.01.2020 um 10:10 schrieb Anshuman Khandual : >> >>  >> >>> On 01/10/2020 02:12 PM, David Hildenbrand wrote: >>>> On 10.01.20 04:09, Anshuman Khandual wrote: >>>> Currently there are two interfaces to initiate memory range hot removal i.e >>>> remove_memory() and __remove_memory() which then calls try_remove_memory(). >>>> Platform gets called with arch_remove_memory() to tear down required kernel >>>> page tables and other arch specific procedures. But there are platforms >>>> like arm64 which might want to prevent removal of certain specific memory >>>> ranges irrespective of their present usage or movability properties. >>> >>> Why? Is this only relevant for boot memory? I hope so, otherwise the >>> arch code needs fixing IMHO. >> >> Right, it is relevant only for the boot memory on arm64 platform. But this >> new arch callback makes it flexible to reject any given memory range. >> >>> >>> If it's only boot memory, we should disallow offlining instead via a >>> memory notifier - much cleaner. >> >> Dont have much detail understanding of MMU notifier mechanism but from some >> initial reading, it seems like we need to have a mm_struct for a notifier >> to monitor various events on the page table. Just wondering how a physical >> memory range like boot memory can be monitored because it can be used both >> for for kernel (init_mm) or user space process at same time. Is there some >> mechanism we could do this ? >> >>> >>>> >>>> Current arch call back arch_remove_memory() is too late in the process to >>>> abort memory hot removal as memory block devices and firmware memory map >>>> entries would have already been removed. Platforms should be able to abort >>>> the process before taking the mem_hotplug_lock with mem_hotplug_begin(). >>>> This essentially requires a new arch callback for memory range validation. >>> >>> I somewhat dislike this very much. Memory removal should never fail if >>> used sanely. See e.g., __remove_memory(), it will BUG() whenever >>> something like that would strike. >>> >>>> >>>> This differentiates memory range validation between memory hot add and hot >>>> remove paths before carving out a new helper check_hotremove_memory_range() >>>> which incorporates a new arch callback. This call back provides platforms >>>> an opportunity to refuse memory removal at the very onset. In future the >>>> same principle can be extended for memory hot add path if required. >>>> >>>> Platforms can choose to override this callback in order to reject specific >>>> memory ranges from removal or can just fallback to a default implementation >>>> which allows removal of all memory ranges. >>> >>> I suspect we want really want to disallow offlining instead. E.g., I >> >> If boot memory pages can be prevented from being offlined for sure, then it >> would indirectly definitely prevent hot remove process as well. >> >>> remember s390x does that with certain areas needed for dumping/kexec. >> >> Could not find any references to mmu_notifier in arch/s390 or any other arch >> for that matter apart from KVM (which has an user space component), could you >> please give some pointers ? > > Memory (hotplug) notifier, not MMU notifier :) They are so similarly named :) > > Not on my notebook right now, grep for MEM_GOING_OFFLINE, that should be it. > Got it, thanks ! But we will still need boot memory enumeration via MEMBLOCK_BOOT to reject affected offline requests in the callback.