[PATCH v1 06/18] mm: make folio_mapcount() return 0 for small typed folios

We already handle it properly for large folios. Let's also return "0"
for small typed folios, like page_mapcount() currently would.

Consequently, folio_mapcount() will never return negative values for
typed folios, but may return negative values for underflows.

Signed-off-by: David Hildenbrand <[email protected]>
---
 include/linux/mm.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index daf687f0e8e5..d453232bba62 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1260,12 +1260,19 @@ static inline int folio_large_mapcount(const struct folio *folio)
  * references the entire folio counts exactly once, even when such special
  * page table entries are comprised of multiple ordinary page table entries.
  *
+ * Will report 0 for pages which cannot be mapped into userspace, such as
+ * slab, page tables and similar.
+ *
  * Return: The number of times this folio is mapped.
  */
 static inline int folio_mapcount(const struct folio *folio)
 {
-	if (likely(!folio_test_large(folio)))
-		return atomic_read(&folio->_mapcount) + 1;
+	int mapcount;
+
+	if (likely(!folio_test_large(folio))) {
+		mapcount = atomic_read(&folio->_mapcount);
+		return page_type_has_type(mapcount) ? 0 : mapcount + 1;
+	}
 	return folio_large_mapcount(folio);
 }
 
-- 
2.44.0