#val_type uint64_t
UFFD_FEATURE_PAGEFAULT_FLAG_WP
UFFD_FEATURE_EVENT_FORK
UFFD_FEATURE_EVENT_REMAP
UFFD_FEATURE_EVENT_REMOVE
UFFD_FEATURE_MISSING_HUGETLBFS
UFFD_FEATURE_MISSING_SHMEM
UFFD_FEATURE_EVENT_UNMAP
UFFD_FEATURE_SIGBUS
UFFD_FEATURE_THREAD_ID