mm/rmap: sanity check that zeropages are not passed to RMAP

Using insert_page() we might have previously ended up passing the zeropage
into rmap code.  Make sure that won't happen again.

Note that we won't check the huge zeropage for now, which might still end
up in RMAP code.

Link: https://lkml.kernel.org/r/20240522125713.775114-4-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/rmap.h

@@ -200,6 +200,9 @@ static inline void __folio_rmap_sanity_checks(struct folio *folio,
 	/* hugetlb folios are handled separately. */
 	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
 
+	/* When (un)mapping zeropages, we should never touch ref+mapcount. */
+	VM_WARN_ON_FOLIO(is_zero_folio(folio), folio);
+
 	/*
 	 * TODO: we get driver-allocated folios that have nothing to do with
 	 * the rmap using vm_insert_page(); therefore, we cannot assume that