Re: [PATCH v1 6/9] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination

[Juan Quintela <quintela@redhat.com>]

David Hildenbrand <david@redhat.com> wrote:
> Currently, when someone (i.e., the VM) accesses discarded parts inside a
> RAMBlock with a RamDiscardManager managing the corresponding mapped memory
> region, postcopy will request migration of the corresponding page from the
> source. The source, however, will never answer, because it refuses to
> migrate such pages with undefined content ("logically unplugged"): the
> pages are never dirty, and get_queued_page() will consequently skip
> processing these postcopy requests.
>
> Especially reading discarded ("logically unplugged") ranges is supposed to
> work in some setups (for example with current virtio-mem), although it
> barely ever happens: still, not placing a page would currently stall the
> VM, as it cannot make forward progress.
>
> Let's check the state via the RamDiscardManager (the state e.g.,
> of virtio-mem is migrated during precopy) and avoid sending a request
> that will never get answered. Place a fresh zero page instead to keep
> the VM working. This is the same behavior that would happen
> automatically without userfaultfd being active, when accessing virtual
> memory regions without populated pages -- "populate on demand".
>
> For now, there are valid cases (as documented in the virtio-mem spec) where
> a VM might read discarded memory; in the future, we will disallow that.
> Then, we might want to handle that case differently, e.g., warning the
> user that the VM seems to be mis-behaving.
>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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