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>

include/linux/mm.h

@@ -825,8 +825,14 @@ static inline int page_mapcount(struct page *page)
 	return atomic_read(&page->_mapcount) + 1;
 }
 
+int folio_mapcount(struct folio *folio);
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-int total_mapcount(struct page *page);
+static inline int total_mapcount(struct page *page)
+{
+	return folio_mapcount(page_folio(page));
+}
+
 int page_trans_huge_mapcount(struct page *page);
 #else
 static inline int total_mapcount(struct page *page)

mm/huge_memory.c

@@ -2465,30 +2465,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	}
 }
 
-int total_mapcount(struct page *page)
-{
-	int i, compound, nr, ret;
-
-	VM_BUG_ON_PAGE(PageTail(page), page);
-
-	if (likely(!PageCompound(page)))
-		return atomic_read(&page->_mapcount) + 1;
-
-	compound = compound_mapcount(page);
-	nr = compound_nr(page);
-	if (PageHuge(page))
-		return compound;
-	ret = compound;
-	for (i = 0; i < nr; i++)
-		ret += atomic_read(&page[i]._mapcount) + 1;
-	/* File pages has compound_mapcount included in _mapcount */
-	if (!PageAnon(page))
-		return ret - compound * nr;
-	if (PageDoubleMap(page))
-		ret -= nr;
-	return ret;
-}
-
 /*
  * This calculates accurately how many mappings a transparent hugepage
  * has (unlike page_mapcount() which isn't fully accurate). This full

mm/util.c

@@ -740,6 +740,39 @@ 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.