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SubjectRe: [PATCH RFC v4 00/13] virtio-mem: paravirtualized memory
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


I'd love to get some feedback on

a) The remaining MM bits from MM folks (especially, patch #6 and #8).
b) The general virtio infrastructure (esp. uapi in patch #2) from virtio

I'm planning to send a proper v1 (!RFC) once I have all necessary MM
acks. In the meanwhile, I will do more testing and minor reworks (e.g.,
fix !CONFIG_NUMA compilation).


David / dhildenb

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