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.

[[email protected]: make folio_mapcount() slightly more efficient]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: John Paul Adrian Glaubitz <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Richard Chang <[email protected]>
Cc: Rich Felker <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yin Fengwei <[email protected]>
Cc: Yoshinori Sato <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 12 insertions, 2 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c7470da5797f..78e583b50e42 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1261,12 +1261,22 @@ 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) + 1;
+		/* Handle page_has_type() pages */
+		if (mapcount < PAGE_MAPCOUNT_RESERVE + 1)
+			mapcount = 0;
+		return mapcount;
+	}
 	return folio_large_mapcount(folio);
 }