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SubjectRe: [PATCH RFC v3 0/9] virtio-mem: paravirtualized memory
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
> -
> 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
> -- 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:
> -
> 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]
> [2]
> [3]
> ---

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 ...)




David / dhildenb

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