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Thu, 16 Apr 2020 10:47:51 -0400 X-MC-Unique: IbturGjoPfaMYwzdzHQ85A-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 93B94192D7AE; Thu, 16 Apr 2020 14:47:49 +0000 (UTC) Received: from [10.36.114.9] (ovpn-114-9.ams2.redhat.com [10.36.114.9]) by smtp.corp.redhat.com (Postfix) with ESMTP id 98DC2A1881; Thu, 16 Apr 2020 14:47:29 +0000 (UTC) Subject: Re: [PATCH 1/3] kexec: Prevent removal of memory in use by a loaded kexec image To: Baoquan He , Andrew Morton References: <20200414064031.GB4247@MiWiFi-R3L-srv> <86e96214-7053-340b-5c1a-ff97fb94d8e0@redhat.com> <20200414092201.GD4247@MiWiFi-R3L-srv> <20200414143912.GE4247@MiWiFi-R3L-srv> <0085f460-b0c7-b25f-36a7-fa3bafaab6fe@redhat.com> <20200415023524.GG4247@MiWiFi-R3L-srv> <18cf6afd-c651-25c7-aca3-3ca3c0e07547@redhat.com> <20200416140247.GA12723@MiWiFi-R3L-srv> <4e1546eb-4416-dc6d-d549-62d1cecccbc8@redhat.com> <20200416143634.GH4247@MiWiFi-R3L-srv> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200416143634.GH4247@MiWiFi-R3L-srv> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200416_074754_941342_BC467513 X-CRM114-Status: GOOD ( 25.80 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: piliu@redhat.com, Anshuman Khandual , Catalin Marinas , Bhupesh Sharma , linuxppc-dev@lists.ozlabs.org, kexec@lists.infradead.org, Russell King - ARM Linux admin , linux-mm@kvack.org, James Morse , "Eric W. Biederman" , Will Deacon , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org >> kexec_walk_memblock() has the option for "kbuf->top_down". Only >> kexec_walk_resources() seems to ignore it. > > Yeah, that top down searching is done in a found low mem area. Means > firstly search an available region bottom up, then put kernel top down > in that region. The reason is our iomem res is linked with singly linked > list. So we can only search bottom up efficiently. > > kexec_load is doing the real top down searching, so kernel will be put > at the top of system ram. I ever tried to change it to support top down > searching for kexec_file_load too with patches, since QE and customers > are often confused with this difference when debugging. > > Andrew may remeber this, he suggested me to change the singly linked list > to doubly linked list for iomem res, then do the top down searching for > kexec_file_load. I tried with some effort, the change introduced too much > code change, I just gave up finally. Well, at least right now this seems to be the right approach (hotplug), lol :) > > http://archive.lwn.net:8080/devicetree/20180718024944.577-1-bhe@redhat.com/ > > I can see that top down searching for kexec can avoid the highly used > low memory region, esp under 4G, for dma, kinds of firmware reserving, > etc. And customers/QE of kexec get used to it. I can change kexec_file_load > to top down too with a simple way if people really complain it. But now, > seems bottom up is not bad too. Ah, I understand the problem. Maybe a simple "optimization" would be to start searching bottom-up from e.g.,2GB/4GB first. If nothing was found, search botoom-up from 0-2GB/4GB etc. > >> >> So I think in case of memblocks (e.g., arm64), this still applies? > > Yeah, aren't you trying to remove it? I haven't read your patches > carefully, maybe I got it wrong. And arm64 even can't support the hot added For arm64 we're still creating memblocks for hotplugged memory, but I guess it's not too hard to stop doing that. > memory being able to recorded into firmware, seems it's not so ready, > won't they change that design in the future? It seems to be incomplete, yes. No idea if it's fixable, no arm64 expert ... >>>>>> - powerpc to filter out all LMBs that can be removed (assuming not all >>>>>> memory corresponds to LMBs that can be removed, otherwise we're in >>>>>> trouble ... :) ) >>>>>> - virtio-mem to filter out all memory it added. >>>>>> - hyper-v to filter out partially backed memory blocks (esp. the last >>>>>> memory block it added and only partially backed it by memory). >>>>>> >>>>>> This would make it work for kexec_file_load(), however, I do wonder how >>>>>> we would want to approach that from userspace kexec-tools when handling >>>>>> it from kexec_load(). >>>>> >>>>> Let's make kexec_file_load work firstly. Since this work is only first >>>>> step to make kexec-ed kernel not break memory hotplug. After kexec >>>>> rebooting, the KASLR may locate kernel into hotpluggable area too. >>>> >>>> Can you elaborate how that would work? >>> >>> Well, boot memory can be hotplugged or not after boot, they are marked >>> in uefi tables, the current kexec doesn't save and pass them into 2nd >>> kenrel, when kexec kernel bootup, it need read them and avoid them to >>> randomize kernel into. >> >> What about e.g., memory hotplugged by ACPI? I would assume, that the >> kexec kernel will not make use of that (IOW detected that) until the >> ACPI driver comes up and re-detects + adds that memory. >> >> Or how would that machinery work in case we have a DIMM hotplugged via ACPI? > > ACPI SRAT is embeded into efi, need read out the rsdp pointer. If we don't > pass the efi, it won't get the SRAT table correctly, if I remember > correctly. Yeah, I remeber kvm guest can get memory hotplugged with > ACPI only, this won't happen on bare metal though. Need check carefully. > I have been using kvm guest with uefi firmwire recently. Yeah, I can imagine that bare metal is different. kvm only uses ACPI. I'm also asking because of virtio-mem. Memory added via virtio-mem is not part of any efi tables or whatsoever. So I assume the kexec kernel will not detect it automatically (good!), instead load the virtio-mem driver and let it add memory back to the system. I should probably play with kexec and virtio-mem once I have some spare cycles ... to find out what's broken and needs to be addressed :) -- Thanks, David / dhildenb _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel