hugetlb: lazy page table copies in fork()

Lazy page table copying at fork time was introduced with commit
d992895 ("[PATCH] Lazy page table copies in fork()").  At the time,
hugetlb was very new and did not support page faulting.  As a result, it
was excluded.  When full page fault support was added for hugetlb, the
exclusion was not removed.

Simply remove the check that prevents lazy copying of hugetlb page tables
at fork.  Of course, like other mappings this only applies to shared
mappings.

Lazy page table copying at fork will be less advantageous for hugetlb
mappings because:
- There are fewer page table entries with hugetlb
- hugetlb pmds can be shared instead of copied

In any case, completely eliminating the copy at fork time should speed
things up.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Kravetz <[email protected]>
Acked-by: Muchun Song <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: James Houghton <[email protected]>
Cc: kernel test robot <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Rolf Eike Beer <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: mjkravetz

mm/memory.c

@@ -1253,7 +1253,7 @@ vma_needs_copy(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
 	if (userfaultfd_wp(dst_vma))
 		return true;
 
-	if (src_vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP))
+	if (src_vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
 		return true;
 
 	if (src_vma->anon_vma)