[PATCH mm-unstable v3 1/6] mm: Add folio_estimated_mapcount()

["Vishal Moola (Oracle)" <vishal.moola@gmail.com>]

folio_estimated_mapcount() takes in a folio and returns the precise
number of times the first subpage of the folio is mapped.

This function aims to provide an estimate for the mapcount of a subpage
within a folio. This is necessary for folio conversions where we care
about the mapcount of a subpage, but not necessarily the whole folio.

This is in contrast to folio_mapcount() which calculates the total
number of the times a folio and all its subpages are mapped.

Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
---
 include/linux/mm.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c9db257f09b3..fdd5b77ac209 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1817,6 +1817,23 @@ static inline size_t folio_size(struct folio *folio)
 	return PAGE_SIZE << folio_order(folio);
 }
 
+/**
+ * folio_estimated_mapcount - Estimate a folio's per-page mapcount.
+ * @folio: The folio.
+ *
+ * folio_estimated_mapcount() aims to serve as a function to efficiently
+ * estimate the number of times each page in a folio is mapped.
+ * This may not be accurate for large folios. If you want exact mapcounts,
+ * look at page_mapcount() or folio_total_mapcount().
+ *
+ * Return: The precise mapcount of the first subpage, meant to estimate
+ * the precise mapcount of any subpage.
+ */
+static inline int folio_estimated_mapcount(struct folio *folio)
+{
+	return page_mapcount(folio_page(folio, 0));
+}
+
 #ifndef HAVE_ARCH_MAKE_PAGE_ACCESSIBLE
 static inline int arch_make_page_accessible(struct page *page)
 {
-- 
2.38.1