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 08C92C33CAE for ; Tue, 14 Jan 2020 02:12:05 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id BB1282084D for ; Tue, 14 Jan 2020 02:12:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BB1282084D 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 569A88E0006; Mon, 13 Jan 2020 21:12:04 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 510D08E0003; Mon, 13 Jan 2020 21:12:04 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 44D438E0006; Mon, 13 Jan 2020 21:12:04 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0231.hostedemail.com [216.40.44.231]) by kanga.kvack.org (Postfix) with ESMTP id 3208D8E0003 for ; Mon, 13 Jan 2020 21:12:04 -0500 (EST) Received: from smtpin20.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with SMTP id B374A8249980 for ; Tue, 14 Jan 2020 02:12:03 +0000 (UTC) X-FDA: 76374614526.20.run96_864f3e1249261 X-HE-Tag: run96_864f3e1249261 X-Filterd-Recvd-Size: 6108 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf44.hostedemail.com (Postfix) with ESMTP for ; Tue, 14 Jan 2020 02:12:02 +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 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: 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 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 : >>>> >>>> =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 re= moval i.e >>>>>> remove_memory() and __remove_memory() which then calls try_remove_= memory(). >>>>>> Platform gets called with arch_remove_memory() to tear down requir= ed kernel >>>>>> page tables and other arch specific procedures. But there are plat= forms >>>>>> like arm64 which might want to prevent removal of certain specific= memory >>>>>> ranges irrespective of their present usage or movability propertie= s. >>>>> >>>>> Why? Is this only relevant for boot memory? I hope so, otherwise th= e >>>>> arch code needs fixing IMHO. >>>> >>>> Right, it is relevant only for the boot memory on arm64 platform. Bu= t 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 fr= om some >>>> initial reading, it seems like we need to have a mm_struct for a not= ifier >>>> to monitor various events on the page table. Just wondering how a ph= ysical >>>> memory range like boot memory can be monitored because it can be use= d both >>>> for for kernel (init_mm) or user space process at same time. Is ther= e some >>>> mechanism we could do this ? >>>> >>>>> >>>>>> >>>>>> Current arch call back arch_remove_memory() is too late in the pro= cess to >>>>>> abort memory hot removal as memory block devices and firmware memo= ry map >>>>>> entries would have already been removed. Platforms should be able = to abort >>>>>> the process before taking the mem_hotplug_lock with mem_hotplug_be= gin(). >>>>>> This essentially requires a new arch callback for memory range val= idation. >>>>> >>>>> 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_memor= y_range() >>>>>> which incorporates a new arch callback. This call back provides pl= atforms >>>>>> an opportunity to refuse memory removal at the very onset. In futu= re 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 imple= mentation >>>>>> 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/kexe= c. >>>> >>>> Could not find any references to mmu_notifier in arch/s390 or any ot= her 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 ME= MBLOCK_BOOT >> to reject affected offline requests in the callback. >=20 > Do you really need that? >=20 > 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. >=20 >=20