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 0FA9AC33CA9 for ; Tue, 14 Jan 2020 02:12:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E1E1B2187F for ; Tue, 14 Jan 2020 02:12:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729546AbgANCMC (ORCPT ); Mon, 13 Jan 2020 21:12:02 -0500 Received: from foss.arm.com ([217.140.110.172]:46700 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729267AbgANCMC (ORCPT ); Mon, 13 Jan 2020 21:12:02 -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 A10B51045; Mon, 13 Jan 2020 18:12:01 -0800 (PST) Received: from [192.168.0.129] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 095483F6C4; Mon, 13 Jan 2020 18:11:54 -0800 (PST) Subject: Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range To: David Hildenbrand , David Hildenbrand Cc: 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> <6b8fb779-31e8-1b63-85a8-9f6c93a04494@arm.com> <19194427-1295-3596-2c2c-463c4adf4f35@redhat.com> From: Anshuman Khandual Message-ID: <78f04711-2ca6-280c-d0c2-ab9eea866e59@arm.com> Date: Tue, 14 Jan 2020 07:43:12 +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: <19194427-1295-3596-2c2c-463c4adf4f35@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 04:07 PM, David Hildenbrand wrote: > On 13.01.20 10:50, Anshuman Khandual wrote: >> >> >> 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. > > Do you really need that? > > We have SECTION_IS_EARLY. You could iterate all involved sections (for > which you are getting notified) and check if any one of these is marked > SECTION_IS_EARLY. then, it was added during boot and not via add_memory(). Seems to be a better approach than adding a new memblock flag. > >