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SubjectRe: [PATCH RFC v4 00/13] virtio-mem: paravirtualized memory
On 29.01.20 10:41, David Hildenbrand wrote:
> On 09.01.20 14:48, David Hildenbrand wrote:
>> On 12.12.19 18:11, David Hildenbrand wrote:
>>> This series is based on latest linux-next. The patches are located at:
>>> virtio-mem-rfc-v4
>>> 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 is limited to x86-64, however, should theoretically work on any
>>> architecture that supports virtio and implements memory hot(un)plug under
>>> Linux - like s390x, powerpc64 and arm64. On x86-64, it is currently
>>> possible to add/remove memory to the system in >= 4MB granularity.
>>> Memory hotplug works very reliably. For memory unplug, there are no
>>> guarantees how much memory can actually get unplugged, it depends on the
>>> setup (especially: fragmentation of (unmovable) memory). 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).
>>> 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.
>>> Memory offlining code is extended to allow drivers to drop their reference
>>> to PG_offline pages when MEM_GOING_OFFLINE, so these pages can be skipped
>>> when offlining memory blocks. This allows to offline memory blocks that
>>> have partially unplugged (allocated e.g., via alloc_contig_range())
>>> 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. Changes RFC v2 -> v3
>>> --------------------------------------------------------------------------
>>> 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. Changes RFC v3 -> RFC v4
>>> --------------------------------------------------------------------------
>>> Only minor things changed, especially nothing on the QEMU + virtio side.
>>> Interresting changes on the Linux driver side are:
>>> - "mm: Allow to offline unmovable PageOffline() pages via
>>> -- Rework to Michals suggestion (allow to isolate all PageOffline() pages
>>> by skipping all PageOffline() pages in has_unmovable_pages(). Fail
>>> offlining later if the pages cannot be offlined/migrated).
>>> - "virtio-mem: Allow to offline partially unplugged memory blocks"
>>> -- Adapt to Michals suggestion on core-mm part.
>>> - "virtio-mem: Better retry handling"
>>> -- Optimize retry intervals
>>> - "virtio-mem: Drop slab objects when unplug continues to fail"
>>> -- Call drop_slab()/drop_slab_node() when unplug keeps failing for a longer
>>> time.
>>> - Multiple cleanups and fixes.
>>> --------------------------------------------------------------------------
>>> 5. 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:
>>> -- There are various idea to limit fragmentation on memory block
>>> granularity. (e.g., ZONE_PREFER_MOVABLE and smart balancing)
>>> -- Allocate memmap from added memory. This way, less unmovable data can
>>> end up on the memory blocks.
>>> - OOM handling, e.g., via an OOM handler.
>>> - Defragmentation
>>> -- Will require a new virtio-mem CMD to exchange plugged<->unplugged blocks
>>> --------------------------------------------------------------------------
>>> 6. Example Usage
>>> --------------------------------------------------------------------------
>>> A very basic QEMU prototype (kept updated) is available at:
>>> virtio-mem
>>> It lacks various features, however, works to test the guest driver side:
>>> - No support for resizable memory regions / memory backends yet
>>> - No protection of unplugged memory (esp., userfaultfd-wp) yet
>>> - No dump/migration/XXX optimizations to skip unplugged memory (and avoid
>>> touching it)
>>> 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 4.1.95 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 4.1.95 monitor - type 'help' for more information
>>> (qemu) qom-set vm1 requested-size 1G
>>> Remove some memory from node 0:
>>> QEMU 4.1.95 monitor - type 'help' for more information
>>> (qemu) qom-set vm0 requested-size 64M
>>> Query the configuration again:
>>> (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
>>> --------------------------------------------------------------------------
>>> 7. 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. I ahve plans for a
>>> virtio-mem defragmentation feature in the future.
>>> 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. 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]
>>> [2]
>>> [3]
>>> Cc: Sebastien Boeuf <>
>>> Cc: Samuel Ortiz <>
>>> Cc: Robert Bradford <>
>>> Cc: Luiz Capitulino <>
>>> David Hildenbrand (13):
>>> 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()
>>> virtio-mem: Paravirtualized memory hotunplug part 2
>>> mm: Allow to offline unmovable PageOffline() pages via
>>> virtio-mem: Allow to offline partially unplugged memory blocks
>>> mm/memory_hotplug: Introduce offline_and_remove_memory()
>>> virtio-mem: Offline and remove completely unplugged memory blocks
>>> virtio-mem: Better retry handling
>>> mm/vmscan: Move count_vm_event(DROP_SLAB) into drop_slab()
>>> mm/vmscan: Export drop_slab() and drop_slab_node()
>>> virtio-mem: Drop slab objects when unplug continues to fail
>> Ping,
>> I'd love to get some feedback on
>> a) The remaining MM bits from MM folks (especially, patch #6 and #8).
> Friendly ping again:
> Can I get some feedback on the two important MM changes in this series
> "[PATCH RFC v4 06/13] mm: Allow to offline unmovable PageOffline() pages
> and
> "[PATCH RFC v4 08/13] mm/memory_hotplug: Introduce
> offline_and_remove_memory()"

Yet another ping.


David / dhildenb

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