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Fri, 10 Jan 2020 08:42:27 +0000 (UTC) Received: from [10.36.117.139] (ovpn-117-139.ams2.redhat.com [10.36.117.139]) by smtp.corp.redhat.com (Postfix) with ESMTP id CC32760C88; Fri, 10 Jan 2020 08:42:22 +0000 (UTC) Subject: Re: [PATCH V11 1/5] mm/hotplug: Introduce arch callback validating the hot remove range To: Anshuman Khandual , 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 Cc: 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: <1578625755-11792-1-git-send-email-anshuman.khandual@arm.com> <1578625755-11792-2-git-send-email-anshuman.khandual@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <1578625755-11792-2-git-send-email-anshuman.khandual@arm.com> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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_memor= y(). > Platform gets called with arch_remove_memory() to tear down required ke= rnel > page tables and other arch specific procedures. But there are platforms > like arm64 which might want to prevent removal of certain specific memo= ry > 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. If it's only boot memory, we should disallow offlining instead via a memory notifier - much cleaner. >=20 > 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 ma= p > entries would have already been removed. Platforms should be able to ab= ort > the process before taking the mem_hotplug_lock with mem_hotplug_begin()= . > This essentially requires a new arch callback for memory range validati= on. 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. >=20 > This differentiates memory range validation between memory hot add and = hot > remove paths before carving out a new helper check_hotremove_memory_ran= ge() > which incorporates a new arch callback. This call back provides platfor= ms > an opportunity to refuse memory removal at the very onset. In future th= e > same principle can be extended for memory hot add path if required. >=20 > Platforms can choose to override this callback in order to reject speci= fic > memory ranges from removal or can just fallback to a default implementa= tion > which allows removal of all memory ranges. I suspect we want really want to disallow offlining instead. E.g., I remember s390x does that with certain areas needed for dumping/kexec. Somebody who added memory via add_memory() should always be able to remove the memory via remove_memory() again. Only boot memory can be treated in a special way, but boot memory is initially always online. --=20 Thanks, David / dhildenb