Re: [PATCH v3 6/7] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 7/30/21 10:52 AM, David Hildenbrand 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.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  migration/postcopy-ram.c | 31 +++++++++++++++++++++++++++----
>  migration/ram.c          | 21 +++++++++++++++++++++
>  migration/ram.h          |  1 +
>  3 files changed, 49 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index 2e9697bdd2..38cdfc09c3 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -671,6 +671,29 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd,
>      return ret;
>  }
>  
> +static int postcopy_request_page(MigrationIncomingState *mis, RAMBlock *rb,
> +                                 ram_addr_t start, uint64_t haddr)
> +{
> +    void *aligned = (void *)(uintptr_t)(haddr & -qemu_ram_pagesize(rb));

  void *aligned = QEMU_ALIGN_PTR_DOWN(haddr, qemu_ram_pagesize(rb)));