[PATCH v2 00/10] virtio-mem: paravirtualized memory

David Hildenbrand
Wed, 11 Mar 2020 17:14:12 -0000

This series is based on latest linux-next. The patches are located at:
    https://github.com/davidhildenbrand/linux.git virtio-mem-v2

I now have acks for all !virtio-mem changes. I'll be happy to get review
feedback, testing reports, etc. for the virtio-mem changes. If there are
no further comments, I guess this is good to go as a v1 soon.

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.

It's currently only enabled for 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 physical memory).

I am currently getting the QEMU side into shape (which will be posted as
RFC soon, see below for a link to the current state). Experimental Kata
support is in the works [4]. Also, a cloud-hypervisor implementation is
under discussion [5].

--------------------------------------------------------------------------
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).

    There are no guarantees what will happen if unplugged memory is
    read/written. Such memory should, in general, not be touched. E.g.,
    even writing might succeed, but the values will simply be discarded at
    random points in time.

    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
--------------------------------------------------------------------------

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.

--------------------------------------------------------------------------
3. Changes v1 -> v2
--------------------------------------------------------------------------

- "virtio-mem: Paravirtualized memory hotplug"
-- Use "__u64" and friends in uapi header
-- Split out ACPI PXM handling
- "virtio-mem: Allow to specify an ACPI PXM as nid"
-- Squash of the ACPI PXM handling and previous "ACPI: NUMA: export
   pxm_to_node"
- "virtio-mem: Paravirtualized memory hotunplug part 2"
-- Squashed previous "mm: Export alloc_contig_range() /
   free_contig_range()"
- "virtio-mem: Allow to offline partially unplugged memory blocks"
-- WARN and dump_page() in case somebody has a reference to an unplugged
   page
- "virtio-mem: Better retry handling"
-- Use retry interval of 5s -> 5m
- Tweaked some patch descriptions

--------------------------------------------------------------------------
4. Future work
--------------------------------------------------------------------------

One of the next TODO things besides the QEMU part is writing a virtio-mem
spec - however, that might still take some time.

virtio-mem extensions (via new feature flags):
- Indicate the guest status (e.g., initialized, working, all memory is
  busy when unplugging, too many memory blocks are offline when plugging,
  etc.)
- Guest-triggered shrinking of the usable region (e.g., whenever the
  highest memory block is removed).
- Exchange of plugged<->unplugged block for defragmentation.

Memory hotplug:
- Reduce the amount of memory resources if that tunes out to be an
  issue. Or try to speed up relevant code paths to deal with many
  resources.
- Allocate vmemmap from added memory.

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 within a memory block first.
- 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 vmemmap from added memory.
- OOM handling, e.g., via an OOM handler/shrinker.
- Defragmentation
- Support for < MAX_ORDER - 1 blocks (esp. pageblock_order)

--------------------------------------------------------------------------
5. Example Usage
--------------------------------------------------------------------------

A QEMU implementation (without protection of unplugged memory, but with
resizable memory regions and optimized migration) is available at (kept
updated):
    https://github.com/davidhildenbrand/qemu.git virtio-mem

Start QEMU with two virtio-mem devices (one per NUMA node):
 $ qemu-system-x86_64 -m 4G,maxmem=204G \
  -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=100G,managed-size=on \
  -device virtio-mem-pci,id=vm0,memdev=mem0,node=0,requested-size=0M \
  -object memory-backend-ram,id=mem1,size=100G,managed-size=on \
  -device virtio-mem-pci,id=vm1,memdev=mem1,node=1,requested-size=1G

Query the configuration:
 QEMU 4.2.50 monitor - type 'help' for more information
 (qemu) info memory-devices
 Memory device [virtio-mem]: "vm0"
   memaddr: 0x140000000
   node: 0
   requested-size: 0
   size: 0
   max-size: 107374182400
   block-size: 2097152
   memdev: /objects/mem0
 Memory device [virtio-mem]: "vm1"
   memaddr: 0x1a40000000
   node: 1
   requested-size: 1073741824
   size: 1073741824
   max-size: 107374182400
   block-size: 2097152
   memdev: /objects/mem1

Add some memory to node 0:
 QEMU 4.2.50 monitor - type 'help' for more information
 (qemu) qom-set vm0 requested-size 1G

Remove some memory from node 1:
 QEMU 4.2.50 monitor - type 'help' for more information
 (qemu) qom-set vm1 requested-size 64M

Query the configuration again:
 QEMU 4.2.50 monitor - type 'help' for more information
 (qemu) info memory-devices
 Memory device [virtio-mem]: "vm0"
   memaddr: 0x140000000
   node: 0
   requested-size: 1073741824
   size: 1073741824
   max-size: 107374182400
   block-size: 2097152
   memdev: /objects/mem0
 Memory device [virtio-mem]: "vm1"
   memaddr: 0x1a40000000
   node: 1
   requested-size: 67108864
   size: 67108864
   max-size: 107374182400
   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 granularity 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. 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 at redhat.com
[3] https://lkml.kernel.org/r/547865a9-d6c2-7140-47e2-5af01e7d761d at redhat.com
[4] https://github.com/kata-containers/documentation/pull/592
[5] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/837

Cc: Sebastien Boeuf <sebastien.boeuf at intel.com>
Cc: Samuel Ortiz <samuel.ortiz at intel.com>
Cc: Robert Bradford <robert.bradford at intel.com>
Cc: Luiz Capitulino <lcapitulino at redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
Cc: teawater <teawaterz at linux.alibaba.com>
Cc: Igor Mammedov <imammedo at redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert at redhat.com>

David Hildenbrand (10):
  virtio-mem: Paravirtualized memory hotplug
  virtio-mem: Allow to specify an ACPI PXM as nid
  virtio-mem: Paravirtualized memory hotunplug part 1
  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
  MAINTAINERS: Add myself as virtio-mem maintainer

 MAINTAINERS                     |    7 +
 drivers/acpi/numa/srat.c        |    1 +
 drivers/virtio/Kconfig          |   18 +
 drivers/virtio/Makefile         |    1 +
 drivers/virtio/virtio_mem.c     | 1910 +++++++++++++++++++++++++++++++
 include/linux/memory_hotplug.h  |    1 +
 include/linux/page-flags.h      |   10 +
 include/uapi/linux/virtio_ids.h |    1 +
 include/uapi/linux/virtio_mem.h |  208 ++++
 mm/memory_hotplug.c             |   81 +-
 mm/page_alloc.c                 |   26 +
 mm/page_isolation.c             |    9 +
 12 files changed, 2263 insertions(+), 10 deletions(-)
 create mode 100644 drivers/virtio/virtio_mem.c
 create mode 100644 include/uapi/linux/virtio_mem.h

--
2.24.1