mm/gup: add missing gup_must_unshare() check to gup_huge_pgd()

All other instances of gup_huge_pXd() perform the unshare check, so update
the PGD-specific function to do so as well.

While checking pgd_write() might seem unusual, this function already
performs such a check via pgd_access_permitted() so this is in line with
the existing implementation.

David said:

: This change makes unshare handling across all GUP-fast variants
: consistent, which is desirable as GUP-fast is complicated enough
: already even when consistent.
: 
: This function was the only one I seemed to have missed (or left out and
: forgot why -- maybe because it's really dead code for now).  The COW
: selftest would identify the problem, so far there was no report. 
: Either the selftest wasn't run on corresponding architectures with that
: hugetlb size, or that code is still dead code and unused by
: architectures.
: 
: the original commit(s) that added unsharing explain why we care about
: these checks:
: 
: a7f2266041 ("mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page")
: 84209e87c6 ("mm/gup: reliable R/O long-term pinning in COW mappings")

Link: https://lkml.kernel.org/r/cb971ac8dd315df97058ea69442ecc007b9a364a.1683381545.git.lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/gup.c

@@ -2755,6 +2755,11 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
 		return 0;
 	}
 
+	if (!pgd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
+		gup_put_folio(folio, refs, flags);
+		return 0;
+	}
+
 	*nr += refs;
 	folio_set_referenced(folio);
 	return 1;