Re: [PATCH v2 1/4] softmmu/physmem: Warn with ram_block_discard_range() on MAP_PRIVATE file mapping

[Juan Quintela <quintela@redhat.com>]

David Hildenbrand <david@redhat.com> wrote:
> ram_block_discard_range() cannot possibly do the right thing in
> MAP_PRIVATE file mappings in the general case.
>
> To achieve the documented semantics, we also have to punch a hole into
> the file, possibly messing with other MAP_PRIVATE/MAP_SHARED mappings
> of such a file.
>
> For example, using VM templating -- see commit b17fbbe55cba ("migration:
> allow private destination ram with x-ignore-shared") -- in combination with
> any mechanism that relies on discarding of RAM is problematic. This
> includes:
> * Postcopy live migration
> * virtio-balloon inflation/deflation or free-page-reporting
> * virtio-mem
>
> So at least warn that there is something possibly dangerous is going on
> when using ram_block_discard_range() in these cases.
>
> Acked-by: Peter Xu <peterx@redhat.com>
> Tested-by: Mario Casquero <mcasquer@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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

(at least we give a warning)

But I wonder if we can do better and test that:
 * Postcopy live migration

   We can check if we are on postcopy, or put a marker so we know that
   postcopy can have problems when started.

 * virtio-balloon inflation/deflation or free-page-reporting

   We can check if we have ever used virtio-balloon.

 * virtio-mem

   We can check if we have used virtio-men


I am just wondering if that is even possible?

Thanks, Juan.