mm: Add folio_mapcount()

This implements the same algorithm as total_mapcount(), which is
transformed into a wrapper function.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 33 insertions, 0 deletions

diff --git a/mm/util.c b/mm/util.c
index 7e43369064c8..b614f423aaa4 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -741,6 +741,39 @@ int __page_mapcount(struct page *page)
 EXPORT_SYMBOL_GPL(__page_mapcount);
 
 /**
+ * folio_mapcount() - Calculate the number of mappings of this folio.
+ * @folio: The folio.
+ *
+ * A large folio tracks both how many times the entire folio is mapped,
+ * and how many times each individual page in the folio is mapped.
+ * This function calculates the total number of times the folio is
+ * mapped.
+ *
+ * Return: The number of times this folio is mapped.
+ */
+int folio_mapcount(struct folio *folio)
+{
+	int i, compound, nr, ret;
+
+	if (likely(!folio_test_large(folio)))
+		return atomic_read(&folio->_mapcount) + 1;
+
+	compound = folio_entire_mapcount(folio);
+	nr = folio_nr_pages(folio);
+	if (folio_test_hugetlb(folio))
+		return compound;
+	ret = compound;
+	for (i = 0; i < nr; i++)
+		ret += atomic_read(&folio_page(folio, i)->_mapcount) + 1;
+	/* File pages has compound_mapcount included in _mapcount */
+	if (!folio_test_anon(folio))
+		return ret - compound * nr;
+	if (folio_test_double_map(folio))
+		ret -= nr;
+	return ret;
+}
+
+/**
  * folio_copy - Copy the contents of one folio to another.
  * @dst: Folio to copy to.
  * @src: Folio to copy from.