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 AF8A4C33CAA for ; Mon, 13 Jan 2020 09:49:02 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 7A6692075B for ; Mon, 13 Jan 2020 09:49:02 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7A6692075B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 1DD128E0005; Mon, 13 Jan 2020 04:49:02 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 165648E0003; Mon, 13 Jan 2020 04:49:02 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 02BE28E0005; Mon, 13 Jan 2020 04:49:01 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0188.hostedemail.com [216.40.44.188]) by kanga.kvack.org (Postfix) with ESMTP id DB7FC8E0003 for ; Mon, 13 Jan 2020 04:49:01 -0500 (EST) Received: from smtpin14.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id 7FB93181AC9B6 for ; Mon, 13 Jan 2020 09:49:01 +0000 (UTC) X-FDA: 76372137282.14.look37_88e7130a03010 X-HE-Tag: look37_88e7130a03010 X-Filterd-Recvd-Size: 5432 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf44.hostedemail.com (Postfix) with ESMTP for ; Mon, 13 Jan 2020 09:49:00 +0000 (UTC) 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: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 01/13/2020 02:44 PM, David Hildenbrand wrote: >=20 >=20 >> Am 13.01.2020 um 10:10 schrieb Anshuman Khandual : >> >> =EF=BB=BF >> >>> 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 remo= val i.e >>>> remove_memory() and __remove_memory() which then calls try_remove_me= mory(). >>>> Platform gets called with arch_remove_memory() to tear down required= kernel >>>> page tables and other arch specific procedures. But there are platfo= rms >>>> like arm64 which might want to prevent removal of certain specific m= emory >>>> 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 notif= ier >> to monitor various events on the page table. Just wondering how a phys= ical >> 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 proce= ss 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_begi= n(). >>>> This essentially requires a new arch callback for memory range valid= ation. >>> >>> I somewhat dislike this very much. Memory removal should never fail i= f >>> 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 a= nd hot >>>> remove paths before carving out a new helper check_hotremove_memory_= range() >>>> which incorporates a new arch callback. This call back provides plat= forms >>>> 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 sp= ecific >>>> memory ranges from removal or can just fallback to a default impleme= ntation >>>> 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, th= en 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 othe= r arch >> for that matter apart from KVM (which has an user space component), co= uld you >> please give some pointers ? >=20 > Memory (hotplug) notifier, not MMU notifier :) They are so similarly named :) >=20 > Not on my notebook right now, grep for MEM_GOING_OFFLINE, that should b= e it. >=20 Got it, thanks ! But we will still need boot memory enumeration via MEMBL= OCK_BOOT to reject affected offline requests in the callback.