mm/huge_memory: Avoid calling pmd_page() on a non-leaf PMD

Calling try_to_unmap() with TTU_SPLIT_HUGE_PMD and a folio that's not
mapped by a PMD causes oopses on arm64 because we now call page_folio()
on an invalid page.  pmd_page() returns a valid page for non-leaf PMDs on
some architectures, so this bug escaped testing before now.  Fix this bug
by delaying the call to pmd_page() until after we know the PMD is a leaf.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=215804
Fixes: af28a988 ("mm/huge_memory: Convert __split_huge_pmd() to take a folio")
Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Zorro Lang <zlang@redhat.com>

mm/huge_memory.c

@@ -2145,15 +2145,14 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	 * pmd against. Otherwise we can end up replacing wrong folio.
 	 */
 	VM_BUG_ON(freeze && !folio);
-	if (folio) {
-		VM_WARN_ON_ONCE(!folio_test_locked(folio));
-		if (folio != page_folio(pmd_page(*pmd)))
-			goto out;
-	}
+	VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
 
 	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
-	    is_pmd_migration_entry(*pmd))
+	    is_pmd_migration_entry(*pmd)) {
+		if (folio && folio != page_folio(pmd_page(*pmd)))
+			goto out;
 		__split_huge_pmd_locked(vma, pmd, range.start, freeze);
+	}
 
 out:
 	spin_unlock(ptl);