mm/vma: do not register private-anon mappings with khugepaged during mmap

We already are registering private-anon VMAs with khugepaged during fault
time, in do_huge_pmd_anonymous_page(). Commit "register suitable readonly
file vmas for khugepaged" moved the khugepaged registration logic from
shmem_mmap to the generic mmap path.

The userspace-visible effect should be this: khugepaged will unnecessarily
scan mm's which haven't yet faulted in. Note that it won't actually
collapse because all PTEs are none.

Now that I think about it, the mm is going to have a file VMA anyways
during fork+exec, so the mm already gets registered during mmap due to the
non-anon case (I *think*), so at least one of either the mmap registration
or fault-time registration is redundant.

Make this logic specific for non-anon mappings.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 613bec092fe7 ("mm: mmap: register suitable readonly file vmas for khugepaged")
Signed-off-by: Dev Jain <[e-mail]>
Cc: "Aneesh Kumar K.V" <[e-mail]>
Cc: Anshuman Khandual <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Liam Howlett <[e-mail]>
Cc: Lorenzo Stoakes <[e-mail]>
Cc: Matthew Wilcow (Oracle) <[e-mail]>
Cc: Ryan Roberts <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Yang Shi <[e-mail]>
Cc: Zi Yan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/vma.c

···
 	 * vma_merge_new_range() calls khugepaged_enter_vma() too, the below
 	 * call covers the non-merge case.
 	 */
-	khugepaged_enter_vma(vma, map->flags);
+	if (!vma_is_anonymous(vma))
+		khugepaged_enter_vma(vma, map->flags);
 	ksm_add_vma(vma);
 	*vmap = vma;
 	return 0;