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

[David Hildenbrand]

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:
>>     https://github.com/davidhildenbrand/linux.git 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
>>    MEM_GOING_OFFLINE"
>> -- 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:
>>     https://github.com/davidhildenbrand/qemu.git 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] https://events.linuxfoundation.org/wp-content/uploads/2017/12/virtio-mem-Paravirtualized-Memory-David-Hildenbrand-Red-Hat-1.pdf
>> [2] https://lkml.kernel.org/r/20190919142228.5483-1-david@xxxxxxxxxx
>> [3] https://lkml.kernel.org/r/547865a9-d6c2-7140-47e2-5af01e7d761d@xxxxxxxxxx
>>
>> Cc: Sebastien Boeuf  <sebastien.boeuf@xxxxxxxxx>
>> Cc: Samuel Ortiz <samuel.ortiz@xxxxxxxxx>
>> Cc: Robert Bradford <robert.bradford@xxxxxxxxx>
>> Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
>>
>> 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
>>     MEM_GOING_OFFLINE
>>   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
via MEM_GOING_OFFLINE"

and

"[PATCH RFC v4 08/13] mm/memory_hotplug: Introduce
offline_and_remove_memory()"

-- 
Thanks,

David / dhildenb