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migration: Pre-fault memory before starting background snasphot
This commit solves the issue with userfault_fd WP feature that background snapshot is based on. For any never poluated or discarded memory page, the UFFDIO_WRITEPROTECT ioctl() would skip updating PTE for that page, thereby loosing WP setting for it. So we need to pre-fault pages for each RAM block to be protected before making a userfault_fd wr-protect ioctl(). Fixes: 278e2f5 (migration: support UFFD write fault processing in ram_save_iterate()) Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com> Reported-by: David Hildenbrand <david@redhat.com> Reviewed-by: David Hildenbrand <david@redhat.com> Message-Id: <20210401092226.102804-4-andrey.gruzdev@virtuozzo.com> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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