Re: [PATCH v1 9/9] migration/ram: Handle RAMBlocks with a RamDiscardManager on background snapshots

[Juan Quintela]

David Hildenbrand <david@redhat.com> wrote:
> We already don't ever migrate memory that corresponds to discarded ranges
> as managed by a RamDiscardManager responsible for the mapped memory region
> of the RAMBlock.
>
> virtio-mem uses this mechanism to logically unplug parts of a RAMBlock.
> Right now, we still populate zeropages for the whole usable part of the
> RAMBlock, which is undesired because:
>
> 1. Even populating the shared zeropage will result in memory getting
>    consumed for page tables.
> 2. Memory backends without a shared zeropage (like hugetlbfs and shmem)
>    will populate an actual, fresh page, resulting in an unintended
>    memory consumption.
>
> Discarded ("logically unplugged") parts have to remain discarded. As
> these pages are never part of the migration stream, there is no need to
> track modifications via userfaultfd WP reliably for these parts.
>
> Further, any writes to these ranges by the VM are invalid and the
> behavior is undefined.
>
> Note that Linux only supports userfaultfd WP on private anonymous memory
> for now, which usually results in the shared zeropage getting populated.
> The issue will become more relevant once userfaultfd WP supports shmem
> and hugetlb.
>
> Acked-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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