[PATCH v1 01/18] mm: allow for detecting underflows with page_mapcount() again

[David Hildenbrand]

Commit 53277bcf126d ("mm: support page_mapcount() on page_has_type()
pages") made it impossible to detect mapcount underflows by treating
any negative raw mapcount value as a mapcount of 0.

We perform such underflow checks in zap_present_folio_ptes() and
zap_huge_pmd(), which would currently no longer trigger.

Let's check against PAGE_MAPCOUNT_RESERVE instead by using
page_type_has_type(), like page_has_type() would, so we can still catch
some underflows.

Fixes: 53277bcf126d ("mm: support page_mapcount() on page_has_type() pages")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/mm.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ef34cf54c14f..0fb8a40f82dd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1229,11 +1229,10 @@ static inline void page_mapcount_reset(struct page *page)
  */
 static inline int page_mapcount(struct page *page)
 {
-	int mapcount = atomic_read(&page->_mapcount) + 1;
+	int mapcount = atomic_read(&page->_mapcount);
 
 	/* Handle page_has_type() pages */
-	if (mapcount < 0)
-		mapcount = 0;
+	mapcount = page_type_has_type(mapcount) ? 0 : mapcount + 1;
 	if (unlikely(PageCompound(page)))
 		mapcount += folio_entire_mapcount(page_folio(page));
 
-- 
2.44.0