Re: [PATCH v5 8/8] virtio-mem: Proper support for preallocation with migration

[Juan Quintela <quintela@redhat.com>]

David Hildenbrand <david@redhat.com> wrote:
> Ordinary memory preallocation runs when QEMU starts up and creates the
> memory backends, before processing the incoming migration stream. With
> virtio-mem, we don't know which memory blocks to preallocate before
> migration started. Now that we migrate the virtio-mem bitmap early, before
> migrating any RAM content, we can safely preallocate memory for all plugged
> memory blocks before migrating any RAM content.
>
> This is especially relevant for the following cases:
>
> (1) User errors
>
> With hugetlb/files, if we don't have sufficient backend memory available on
> the migration destination, we'll crash QEMU (SIGBUS) during RAM migration
> when running out of backend memory. Preallocating memory before actual
> RAM migration allows for failing gracefully and informing the user about
> the setup problem.
>
> (2) Excluded memory ranges during migration
>
> For example, virtio-balloon free page hinting will exclude some pages
> from getting migrated. In that case, we won't crash during RAM
> migration, but later, when running the VM on the destination, which is
> bad.
>
> To fix this for new QEMU machines that migrate the bitmap early,
> preallocate the memory early, before any RAM migration. Warn with old
> QEMU machines.
>
> Getting postcopy right is a bit tricky, but we essentially now implement
> the same (problematic) preallocation logic as ordinary preallocation:
> preallocate memory early and discard it again before precopy starts. During
> ordinary preallocation, discarding of RAM happens when postcopy is advised.
> As the state (bitmap) is loaded after postcopy was advised but before
> postcopy starts listening, we have to discard memory we preallocated
> immediately again ourselves.
>
> Note that nothing (not even hugetlb reservations) guarantees for postcopy
> that backend memory (especially, hugetlb pages) are still free after they
> were freed ones while discarding RAM. Still, allocating that memory at
> least once helps catching some basic setup problems.
>
> Before this change, trying to restore a VM when insufficient hugetlb
> pages are around results in the process crashing to to a "Bus error"
> (SIGBUS). With this change, QEMU fails gracefully:
>
>   qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address
>   qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-mem-device-early'
>   qemu-system-x86_64: load of migration failed: Cannot allocate memory
>
> And we can even introspect the early migration data, including the
> bitmap:
>   $ ./scripts/analyze-migration.py -f STATEFILE
>   {
>   "ram (2)": {
>       "section sizes": {
>           "0000:00:03.0/mem0": "0x0000000780000000",
>           "0000:00:04.0/mem1": "0x0000000780000000",
>           "pc.ram": "0x0000000100000000",
>           "/rom@etc/acpi/tables": "0x0000000000020000",
>           "pc.bios": "0x0000000000040000",
>           "0000:00:02.0/e1000.rom": "0x0000000000040000",
>           "pc.rom": "0x0000000000020000",
>           "/rom@etc/table-loader": "0x0000000000001000",
>           "/rom@etc/acpi/rsdp": "0x0000000000001000"
>       }
>   },
>   "0000:00:03.0/virtio-mem-device-early (51)": {
>       "tmp": "00 00 00 01 40 00 00 00 00 00 00 07 80 00 00 00 00 00 00 00 00 20 00 00 00 00 00 00",
>       "size": "0x0000000040000000",
>       "bitmap": "ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff [...]
>   },
>   "0000:00:04.0/virtio-mem-device-early (53)": {
>       "tmp": "00 00 00 08 c0 00 00 00 00 00 00 07 80 00 00 00 00 00 00 00 00 20 00 00 00 00 00 00",
>       "size": "0x00000001fa400000",
>       "bitmap": "ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff [...]
>   },
>   [...]
>
> Reported-by: Jing Qi <jinqi@redhat.com>
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>