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[v3,0/7] migration/ram: Optimize for virtio-mem via RamDiscardManager

Message ID 20210730085249.8246-1-david@redhat.com (mailing list archive)
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Series migration/ram: Optimize for virtio-mem via RamDiscardManager | expand


David Hildenbrand July 30, 2021, 8:52 a.m. UTC
virtio-mem exposes a dynamic amount of memory within RAMBlocks by
coordinating with the VM. Memory within a RAMBlock can either get
plugged and consequently used by the VM, or unplugged and consequently no
longer used by the VM. Logical unplug is realized by discarding the
physical memory backing for virtual memory ranges, similar to memory

However, important difference to virtio-balloon are:

a) A virtio-mem device only operates on its assigned memory region /
   RAMBlock ("device memory")
b) Initially, all device memory is logically unplugged
c) Virtual machines will never accidentally reuse memory that is currently
   logically unplugged. The spec defines most accesses to unplugged memory
   as "undefined behavior" -- except reading unplugged memory, which is
   currently expected to work, but that will change in the future.
d) The (un)plug granularity is in the range of megabytes -- "memory blocks"
e) The state (plugged/unplugged) of a memory block is always known and
   properly tracked.

Whenever memory blocks within the RAMBlock get (un)plugged, changes are
communicated via the RamDiscardManager to other QEMU subsystems, most
prominently vfio which updates the DMA mapping accordingly. "Unplugging"
corresponds to "discarding" and "plugging" corresponds to "populating".

While migrating (precopy/postcopy) that state of such memory blocks cannot
change, as virtio-mem will reject any guest requests that would change
the state of blocks with "busy". We don't want to migrate such logically
unplugged memory, because it can result in an unintended memory consumption
both, on the source (when reading memory from some memory backends) and on
the destination (when writing memory). Further, migration time can be
heavily reduced when skipping logically unplugged blocks and we avoid
populating unnecessary page tables in Linux.

Right now, virtio-mem reuses the free page hinting infrastructure during
precopy to exclude all logically unplugged ("discarded") parts from the
migration stream. However, there are some scenarios that are not handled
properly and need fixing. Further, there are some ugly corner cases in
postcopy code and background snapshotting code that similarly have to
handle such special RAMBlocks.

Let's reuse the RamDiscardManager infrastructure to essentially handle
precopy, postcopy and background snapshots cleanly, which means:

a) In precopy code, fixing up the initial dirty bitmaps (in the RAMBlock
   and e.g., KVM) to exclude discarded ranges.
b) In postcopy code, placing a zeropage when requested to handle a page
   falling into a discarded range -- because the source will never send it.
   Further, fix up the dirty bitmap when overwriting it in recovery mode.
c) In background snapshot code, never populating discarded ranges, not even
   with the shared zeropage, to avoid unintended memory consumption,
   especially in the future with hugetlb and shmem.

Detail: When realizing a virtio-mem devices, it will register the RAM
        for migration via vmstate_register_ram(). Further, it will
        set itself as the RamDiscardManager for the corresponding memory
        region of the RAMBlock via memory_region_set_ram_discard_manager().
        Last but not least, memory device code will actually map the
        memory region into guest physical address space. So migration
        code can always properly identify such RAMBlocks.

Tested with precopy/postcopy on shmem, where even reading unpopulated
memory ranges will populate actual memory and not the shared zeropage.
Tested with background snapshots on anonymous memory, because other
backends are not supported yet with upstream Linux.

Idealy, this should all go via the migration tree.

v2 -> v3:
- "migration/ram: Don't passs RAMState to
-- Added to make the next patch easier to implement
- "migration/ram: Handle RAMBlocks with a RamDiscardManager on the migration
-- Fixup the dirty bitmaps only initially and during postcopy recovery,
   not after every bitmap sync. Also properly clear the dirty bitmaps e.g.,
   in KVM. [Peter]
- "migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the
-- Take care of proper host-page alignment [Peter]

v1 -> v2:
- "migration/ram: Handle RAMBlocks with a RamDiscardManager on the
   migration source"
-- Added a note how it interacts with the clear_bmap and what we might want
   to further optimize in the future when synchronizing bitmaps.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Cc: Marek Kedzierski <mkedzier@redhat.com>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: teawater <teawaterz@linux.alibaba.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta@cloud.ionos.com>

David Hildenbrand (7):
  memory: Introduce replay_discarded callback for RamDiscardManager
  virtio-mem: Implement replay_discarded RamDiscardManager callback
  migration/ram: Don't passs RAMState to
  migration/ram: Handle RAMBlocks with a RamDiscardManager on the
    migration source
  virtio-mem: Drop precopy notifier
  migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the
  migration/ram: Handle RAMBlocks with a RamDiscardManager on background

 hw/virtio/virtio-mem.c         |  92 +++++++++++-------
 include/exec/memory.h          |  21 +++++
 include/hw/virtio/virtio-mem.h |   3 -
 migration/postcopy-ram.c       |  31 +++++-
 migration/ram.c                | 166 +++++++++++++++++++++++++++++----
 migration/ram.h                |   1 +
 softmmu/memory.c               |  11 +++
 7 files changed, 267 insertions(+), 58 deletions(-)