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=-7.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable 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 82C38C4360C for ; Wed, 16 Oct 2019 08:12:42 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 2978321848 for ; Wed, 16 Oct 2019 08:12:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2978321848 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id CC6388E0005; Wed, 16 Oct 2019 04:12:41 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C4DC18E0001; Wed, 16 Oct 2019 04:12:41 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B3C6F8E0005; Wed, 16 Oct 2019 04:12:41 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0099.hostedemail.com [216.40.44.99]) by kanga.kvack.org (Postfix) with ESMTP id 85D768E0001 for ; Wed, 16 Oct 2019 04:12:41 -0400 (EDT) Received: from smtpin02.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id 242EA40F0 for ; Wed, 16 Oct 2019 08:12:41 +0000 (UTC) X-FDA: 76048931322.02.pets88_5194dff2caa37 X-HE-Tag: pets88_5194dff2caa37 X-Filterd-Recvd-Size: 20254 Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by imf36.hostedemail.com (Postfix) with ESMTP for ; Wed, 16 Oct 2019 08:12:40 +0000 (UTC) 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 mx1.redhat.com (Postfix) with ESMTPS id A8D58801685; Wed, 16 Oct 2019 08:12:38 +0000 (UTC) Received: from [10.36.117.237] (ovpn-117-237.ams2.redhat.com [10.36.117.237]) by smtp.corp.redhat.com (Postfix) with ESMTP id 424FB60852; Wed, 16 Oct 2019 08:12:23 +0000 (UTC) Subject: Re: [PATCH RFC v3 0/9] virtio-mem: paravirtualized memory To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, virtualization@lists.linux-foundation.org, Andrea Arcangeli , Alexander Duyck , Alexander Potapenko , Andrew Morton , Andrey Ryabinin , Anshuman Khandual , Anthony Yznaga , Dan Williams , Dave Young , Igor Mammedov , Ira Weiny , Jason Gunthorpe , Jason Wang , Johannes Weiner , Juergen Gross , Len Brown , Matthew Wilcox , Mauro Carvalho Chehab , Mel Gorman , "Michael S. Tsirkin" , Michal Hocko , Michal Hocko , Mike Rapoport , Mike Rapoport , Minchan Kim , Oscar Salvador , Oscar Salvador , Pavel Tatashin , Pavel Tatashin , Pingfan Liu , Qian Cai , "Rafael J. Wysocki" , Stefan Hajnoczi , Stephen Rothwell , Vlastimil Babka , Wei Yang , Wei Yang , Yang Shi , Yu Zhao References: <20190919142228.5483-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <237f70f5-76f7-59b3-072c-39578ba6f43a@redhat.com> Date: Wed, 16 Oct 2019 10:12:22 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 MIME-Version: 1.0 In-Reply-To: <20190919142228.5483-1-david@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.67]); Wed, 16 Oct 2019 08:12:39 +0000 (UTC) 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 19.09.19 16:22, David Hildenbrand wrote: > Long time no RFC! I finally had time to get the next version of the Linux > driver side of virtio-mem into shape, incorporating ideas and feedback from > previous discussions. > > This RFC is based on the series currently on the mm list: > - [PATCH 0/3] Remove __online_page_set_limits() > - [PATCH v1 0/3] mm/memory_hotplug: Export generic_online_page() > - [PATCH v4 0/8] mm/memory_hotplug: Shrink zones before removing memory > The current state (kept updated) is available on > - https://github.com/davidhildenbrand/linux/tree/virtio-mem > > The basic idea of virtio-mem is to provide a flexible, > cross-architecture memory hot(un)plug solution that avoids many limitations > imposed by existing technologies, architectures, and interfaces. More > details can be found below and in linked material. > > This RFC was only tested on x86-64, however, should theoretically work > on any Linux architecture that implements memory hot(un)plug - like > s390x. On x86-64, it is currently possible to add/remove memory to the > system in >= 4MB granularity. Memory hotplug works very reliable. For > memory unplug, there are no guarantees how much memory can actually get > unplugged, it depends on the setup. I have plans to improve that in the > future. > > -------------------------------------------------------------------------- > 1. virtio-mem > -------------------------------------------------------------------------- > > The basic idea behind virtio-mem was presented at KVM Forum 2018. The > slides can be found at [1]. The previous RFC can be found at [2]. The > first RFC can be found at [3]. However, the concept evolved over time. The > KVM Forum slides roughly match the current design. > > Patch #2 ("virtio-mem: Paravirtualized memory hotplug") contains quite some > information, especially in "include/uapi/linux/virtio_mem.h": > > Each virtio-mem device manages a dedicated region in physical address > space. Each device can belong to a single NUMA node, multiple devices > for a single NUMA node are possible. A virtio-mem device is like a > "resizable DIMM" consisting of small memory blocks that can be plugged > or unplugged. The device driver is responsible for (un)plugging memory > blocks on demand. > > Virtio-mem devices can only operate on their assigned memory region in > order to (un)plug memory. A device cannot (un)plug memory belonging to > other devices. > > The "region_size" corresponds to the maximum amount of memory that can > be provided by a device. The "size" corresponds to the amount of memory > that is currently plugged. "requested_size" corresponds to a request > from the device to the device driver to (un)plug blocks. The > device driver should try to (un)plug blocks in order to reach the > "requested_size". It is impossible to plug more memory than requested. > > The "usable_region_size" represents the memory region that can actually > be used to (un)plug memory. It is always at least as big as the > "requested_size" and will grow dynamically. It will only shrink when > explicitly triggered (VIRTIO_MEM_REQ_UNPLUG). > > Memory in the usable region can usually be read, however, there are no > guarantees. It can happen that the device cannot process a request, > because it is busy. The device driver has to retry later. > > Usually, during system resets all memory will get unplugged, so the > device driver can start with a clean state. However, in specific > scenarios (if the device is busy) it can happen that the device still > has memory plugged. The device driver can request to unplug all memory > (VIRTIO_MEM_REQ_UNPLUG) - which might take a while to succeed if the > device is busy. > > -------------------------------------------------------------------------- > 2. Linux Implementation > -------------------------------------------------------------------------- > > This RFC reuses quite some existing MM infrastructure, however, has to > expose some additional functionality. > > Memory blocks (e.g., 128MB) are added/removed on demand. Within these > memory blocks, subblocks (e.g., 4MB) are plugged/unplugged. The sizes > depend on the target architecture, MAX_ORDER + pageblock_order, and > the block size of a virtio-mem device. > > add_memory()/try_remove_memory() is used to add/remove memory blocks. > virtio-mem will not online memory blocks itself. This has to be done by > user space, or configured into the kernel > (CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE). virtio-mem will only unplug memory > that was online to the ZONE_NORMAL. Memory is suggested to be onlined to > the ZONE_NORMAL for now. > > The memory hotplug notifier is used to properly synchronize against > onlining/offlining of memory blocks and to track the states of memory > blocks (including the zone memory blocks are onlined to). Locking is > done similar to the PPC CMA implementation. > > The set_online_page() callback is used to keep unplugged subblocks > of a memory block fake-offline when onlining the memory block. > generic_online_page() is used to fake-online plugged subblocks. This > handling is similar to the Hyper-V balloon driver. > > PG_offline is used to mark unplugged subblocks as offline, so e.g., > dumping tools (makedumpfile) will skip these pages. This is similar to > other balloon drivers like virtio-balloon and Hyper-V. > > PG_offline + reference count of 0 [new] is now also used to mark pages as > a "skip" when offlining memory blocks. This allows to offline memory blocks > that have partially unplugged subblocks - or are completely unplugged. > > alloc_contig_range()/free_contig_range() [now exposed] is used to > unplug/plug subblocks of memory blocks the are already exposed to Linux. > > offline_and_remove_memory() [new] is used to offline a fully unplugged > memory block and remove it from Linux. > > > A lot of additional information can be found in the separate patches and > as comments in the code itself. > > -------------------------------------------------------------------------- > 3. Major changes since last RFC > -------------------------------------------------------------------------- > > A lot of things changed, especially also on the QEMU + virtio side. The > biggest changes on the Linux driver side are: > - Onlining/offlining of subblocks is now emulated on top of memory blocks. > set_online_page()+alloc_contig_range()+free_contig_range() is now used > for that. Core MM does not have to be modified and will continue to > online/offline full memory blocks. > - Onlining/offlining of memory blocks is no longer performed by virtio-mem. > - Pg_offline is upstream and can be used. It is also used to allow > offlining of partially unplugged memory blocks. > - Memory block states + subblocks are now tracked more space-efficient. > - Proper kexec(), kdump(), driver unload, driver reload, ZONE_MOVABLE, ... > handling. > > -------------------------------------------------------------------------- > 4. Future work > -------------------------------------------------------------------------- > > The separate patches contain a lot of future work items. One of the next > steps is to make memory unplug more likely to succeed - currently, there > are no guarantees on how much memory can get unplugged again. I have > various ideas on how to limit fragmentation of all memory blocks that > virtio-mem added. > > Memory hotplug: > - Reduce the amount of memory resources if that turnes out to be an > issue. Or try to speed up relevant code paths to deal with many > resources. > - Allocate the vmemmap from the added memory. Makes hotplug more likely > to succeed, the vmemmap is stored on the same NUMA node and that > unmovable memory will later not hinder unplug. > > Memory hotunplug: > - Performance improvements: > -- Sense (lockless) if it make sense to try alloc_contig_range() at all > before directly trying to isolate and taking locks. > -- Try to unplug bigger chunks if possible first. > -- Identify free areas first, that don't have to be evacuated. > - Make unplug more likely to succeed: > -- The "issue" is that in the ZONE_NORMAL, the buddy will randomly > allocate memory. Only pageblocks somewhat limit fragmentation, > however we would want to limit fragmentation on subblock granularity > and even memory block granularity. One idea is to have a new > ZONE_PREFER_MOVABLE. Memory blocks will then be onlined to ZONE_NORMAL > / ZONE_PREFER_MOVABLE in a certain ratio per node (e.g., > 1:4). This makes unplug of quite some memory likely to succeed in most > setups. ZONE_PREFER_MOVABLE is then a mixture of ZONE_NORMAL and > ZONE_MOVABlE. Especially, movable data can end up on that zone, but > only if really required - avoiding running out of memory on ZONE > imbalances. The zone fallback order would be > MOVABLE=>PREFER_MOVABLE=>HIGHMEM=>NORMAL=>PREFER_MOVABLE=>DMA32=>DMA > -- Allocate memmap from added memory. This way, less unmovable data can > end up on the memory blocks. > -- Call drop_slab() before trying to unplug. Eventually shrink other > caches. > - Better retry handling in case memory is busy. We certainly don't want > to try for ever in a short interval to try to get some memory back. > - OOM handling, e.g., via an OOM handler. > > -------------------------------------------------------------------------- > 5. Example Usage > -------------------------------------------------------------------------- > > A very basic QEMU prototype (kept updated) is available at: > - https://github.com/davidhildenbrand/qemu/tree/virtio-mem > > It lacks various features, however works to test the guest driver side: > - No support for resizable memory regions / memory backends > - No protection of unplugged memory (esp., userfaultfd-wp) > - No dump/migration/XXX optimizations to skip over unplugged memory > > Start QEMU with two virtio-mem devices (one per NUMA node): > $ qemu-system-x86_64 -m 4G,maxmem=20G \ > -smp sockets=2,cores=2 \ > -numa node,nodeid=0,cpus=0-1 -numa node,nodeid=1,cpus=2-3 \ > [...] > -object memory-backend-ram,id=mem0,size=8G \ > -device virtio-mem-pci,id=vm0,memdev=mem0,node=0,requested-size=128M \ > -object memory-backend-ram,id=mem1,size=8G \ > -device virtio-mem-pci,id=vm1,memdev=mem1,node=1,requested-size=80M > > Query the configuration: > QEMU 4.1.50 monitor - type 'help' for more information > (qemu) info memory-devices > Memory device [virtio-mem]: "vm0" > memaddr: 0x140000000 > node: 0 > requested-size: 134217728 > size: 134217728 > max-size: 8589934592 > block-size: 2097152 > memdev: /objects/mem0 > Memory device [virtio-mem]: "vm1" > memaddr: 0x340000000 > node: 1 > requested-size: 83886080 > size: 83886080 > max-size: 8589934592 > block-size: 2097152 > memdev: /objects/mem1 > > Add some memory to node 1: > QEMU 4.1.50 monitor - type 'help' for more information > (qemu) qom-set vm1 requested-size 1G > > Remove some memory from node 0: > QEMU 4.1.50 monitor - type 'help' for more information > (qemu) qom-set vm0 requested-size 64M > > Query the configuration: > (qemu) info memory-devices > Memory device [virtio-mem]: "vm0" > memaddr: 0x140000000 > node: 0 > requested-size: 67108864 > size: 67108864 > max-size: 8589934592 > block-size: 2097152 > memdev: /objects/mem0 > Memory device [virtio-mem]: "vm1" > memaddr: 0x340000000 > node: 1 > requested-size: 1073741824 > size: 1073741824 > max-size: 8589934592 > block-size: 2097152 > memdev: /objects/mem1 > > -------------------------------------------------------------------------- > 6. Q/A > -------------------------------------------------------------------------- > > Q: Why add/remove parts ("subblocks") of memory blocks/sections? > A: Flexibility (section size depends on the architecture) - e.g., some > architectures have a section size of 2GB. Also, the memory block size > is variable (e.g., on x86-64). I want to avoid any such restrictions. > Some use cases want to add/remove memory in smaller granularities to a > VM (e.g., the Hyper-V balloon also implements this) - especially smaller > VMs like used for kata containers. Also, on memory unplug, it is more > reliable to free-up and unplug multiple small chunks instead > of one big chunk. E.g., if one page of a DIMM is either unmovable or > pinned, the DIMM can't get unplugged. This approach is basically a > compromise between DIMM-based memory hot(un)plug and balloon > inflation/deflation, which works mostly on page granularity. > > Q: Why care about memory blocks? > A: They are the way to tell user space about new memory. This way, > memory can get onlined/offlined by user space. Also, e.g., kdump > relies on udev events to reload kexec when memory blocks are > onlined/offlined. Memory blocks are the "real" memory hot(un)plug > granularity. Everything that's smaller has to be emulated "on top". > > Q: Won't memory unplug of subblocks fragment memory? > A: Yes and no. Unplugging e.g., >=4MB subblocks on x86-64 will not really > fragment memory like unplugging random pages like a balloon driver does. > Buddy merging will not be limited. However, any allocation that requires > bigger consecutive memory chunks (e.g., gigantic pages) might observe > the fragmentation. Possible solutions: Allocate gigantic huge pages > before unplugging memory, don't unplug memory, combine virtio-mem with > DIMM based memory or bigger initial memory. Remember, a virtio-mem > device will only unplug on the memory range it manages, not on other > DIMMs. Unplug of single memory blocks will result in similar > fragmentation in respect to gigantic huge pages. > > Q: How reliable is memory unplug? > A: There are no guarantees on how much memory can get unplugged > again. However, it is more likely to find 4MB chunks to unplug than > e.g., 128MB chunks. If memory is terribly fragmented, there is nothing > we can do - for now. I consider memory hotplug the first primary use > of virtio-mem. Memory unplug might usually work, but we want to improve > the performance and the amount of memory we can actually unplug later. > > Q: Why not unplug from the ZONE_MOVABLE? > A: Unplugged memory chunks are unmovable. Unmovable data must not end up > on the ZONE_MOVABLE - similar to gigantic pages - they will never be > allocated from ZONE_MOVABLE. Teaching MM to move unplugged chunks within > a device might be problematic and will require a new guest->hypervisor > command to move unplugged chunks. virtio-mem added memory can be onlined > to the ZONE_MOVABLE, but subblocks will not get unplugged from it. > > Q: How big should the initial (!virtio-mem) memory of a VM be? > A: virtio-mem memory will not go to the DMA zones. So to avoid running out > of DMA memory, I suggest something like 2-3GB on x86-64. But many > VMs can most probably deal with less DMA memory - depends on the use > case. > > [1] https://events.linuxfoundation.org/wp-content/uploads/2017/12/virtio-mem-Paravirtualized-Memory-David-Hildenbrand-Red-Hat-1.pdf > [2] https://lwn.net/Articles/755423/ > [3] https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg03870.html > > --- > Gentle ping. I want to continue working on this. I need feedback from MM people on ... > David Hildenbrand (9): > ACPI: NUMA: export pxm_to_node > virtio-mem: Paravirtualized memory hotplug > virtio-mem: Paravirtualized memory hotunplug part 1 > mm: Export alloc_contig_range() / free_contig_range() ... this patch, if it is okay to export alloc_contig_range() / free_contig_range() (see next patch how it is used) > virtio-mem: Paravirtualized memory hotunplug part 2 > mm: Allow to offline PageOffline() pages with a reference count of 0 ... this patch, to see if we can use "PageOffline() + refcount == 0" as a way to allow offlining memory with unplugged pieces. (see next patch how this is used) > virtio-mem: Allow to offline partially unplugged memory blocks > mm/memory_hotplug: Introduce offline_and_remove_memory() ... I assume this patch is not that debatable. > virtio-mem: Offline and remove completely unplugged memory blocks So yeah, please feedback on "mm: Export alloc_contig_range() / free_contig_range()" and "mm: Allow to offline PageOffline() pages with a reference count of 0", because they are the essential pieces apart from what we already have (generic_online_page(), memory notifiers ...) Thanks! -- Thanks, David / dhildenb