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

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Date: Tue,  9 Apr 2024 21:22:49 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	linux-doc@...r.kernel.org,
	cgroups@...r.kernel.org,
	linux-sh@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Peter Xu <peterx@...hat.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Yin Fengwei <fengwei.yin@...el.com>,
	Yang Shi <shy828301@...il.com>,
	Zi Yan <ziy@...dia.com>,
	Jonathan Corbet <corbet@....net>,
	Hugh Dickins <hughd@...gle.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Rich Felker <dalias@...c.org>,
	John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Miaohe Lin <linmiaohe@...wei.com>,
	Naoya Horiguchi <naoya.horiguchi@....com>,
	Richard Chang <richardycc@...gle.com>
Subject: [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 <david@...hat.com>
---
 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