[PULL 4/6] migration: Pre-fault memory before starting background snasphot

From: Andrey Gruzdev 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

[PULL 4/6] migration: Pre-fault memory before starting background snasphot

From: Andrey Gruzdev 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