This flag to vm_page_grab_valid will cause it to return VM_PAGER_FAIL rather than create a zero page, like VM_ALLOC_NOCREAT. However, unlike VM_ALLOC_NOCREAT, it will not skip page-in of a (partially) invalid page.
This can be used when the kernel is traversing a partially demand-filled/ZFoD swath of the address space without fully emulating faults at each page (and so being seen as demanding zero fill). In particular, if a vm_object has a backing_object, it's not safe to skip over gaps using vm_page_find_least; vm_page_grab_valid (and vm_page_lookup therein) must be used to probe for backing pages in shadowed objects.
This is a lighter-weight change than an alternative option of a vm_page_find_backed_least or similar that produced the pindex of the next non-ZFoD page taking the entire backing hierarchy into account.
If the use of the last VM_ALLOC flag bit is problematic, I suspect this could be given the same value as VM_ALLOC_NORECLAIM, which claims (and appears) to be used only by vm_page_alloc_contig.