What are the semantics of VM_UFFD_MISSING?

Question

I'm reading through the Linux kernel's page fault handler (linux-4.14.198), and noticed the following block of code:

        /* Deliver the page fault to userland, check inside PT lock */
        if (userfaultfd_missing(vma)) {
            pte_unmap_unlock(vmf->pte, vmf->ptl);
            return handle_userfault(vmf, VM_UFFD_MISSING);
        }

The definition of userfaultfd_missing is that vm_flags includes VM_UFFD_MISSING. However, the only comment on this flag is the cryptic missing pages tracking comment on the macro definition.

What does this flag mean, and what exactly is "userfault" handling?