Re: [PATCH v1 0/9] migration/ram: Optimize for virtio-mem via RamDiscardManager

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* David Hildenbrand (david@redhat.com) wrote:
> This series is fully reviewed by Peter and I hope we can get either more
> review feedback or get it merged via the migration tree soonish. Thanks.

Yep, I think that's a full set now; we should take this via migration.

Dave

> ---
> 
> 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
> ballooning.
> 
> 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.
> 
> 
> v5 -> v6:
> - Rebased and added ACKs
> 
> v4 -> v5:
> - "migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the
>    destination"
> -- Use ROUND_DOWN and fix compile warning on 32 bit
> -- Use int128_make64() instead of wrongly int128_get64()
> - "migration: Simplify alignment and alignment checks"
> -- Use ROUND_DOWN where possible instead of QEMU_ALIGN_DOWN and fix
>    compilation warning on 32 bit
> - "migration/ram: Factor out populating pages readable in
>    ram_block_populate_pages()"
> -- Rename functions, add a comment.
> - "migration/ram: Handle RAMBlocks with a RamDiscardManager on background
>    snapshots"
> -- Adjust to changed function names
> 
> v3 -> v4:
> - Added ACKs
> - "migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the
>    destination"
> -- Use QEMU_ALIGN_DOWN() to align to ram pagesize
> - "migration: Simplify alignment and alignment checks"
> -- Added
> - "migration/ram: Factor out populating pages readable in
>    ram_block_populate_pages()"
> -- Added
> - "migration/ram: Handle RAMBlocks with a RamDiscardManager on background
>    snapshots"
> -- Simplified due to factored out code
> 
> v2 -> v3:
> - "migration/ram: Don't passs RAMState to
>    migration_clear_memory_region_dirty_bitmap_*()"
> -- Added to make the next patch easier to implement
> - "migration/ram: Handle RAMBlocks with a RamDiscardManager on the migration
>    source"
> -- 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
>    destination"
> -- 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>
> Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
> 
> David Hildenbrand (9):
>   memory: Introduce replay_discarded callback for RamDiscardManager
>   virtio-mem: Implement replay_discarded RamDiscardManager callback
>   migration/ram: Don't passs RAMState to
>     migration_clear_memory_region_dirty_bitmap_*()
>   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
>     destination
>   migration: Simplify alignment and alignment checks
>   migration/ram: Factor out populating pages readable in
>     ram_block_populate_pages()
>   migration/ram: Handle RAMBlocks with a RamDiscardManager on background
>     snapshots
> 
>  hw/virtio/virtio-mem.c         |  92 ++++++++++-------
>  include/exec/memory.h          |  21 ++++
>  include/hw/virtio/virtio-mem.h |   3 -
>  migration/migration.c          |   6 +-
>  migration/postcopy-ram.c       |  40 ++++++--
>  migration/ram.c                | 180 +++++++++++++++++++++++++++++----
>  migration/ram.h                |   1 +
>  softmmu/memory.c               |  11 ++
>  8 files changed, 284 insertions(+), 70 deletions(-)
> 
> -- 
> 2.31.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK