[PATCH mm-unstable v4 1/6] mm: add folio_estimated_sharers()

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

folio_estimated_sharers() 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 number of sharers of a
folio. This is necessary for folio conversions where we care
about the number of processes that share a folio, but don't necessarily
want to check every single page within that 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 | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 27b34f7730e7..c91bf9cdb3d0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1905,6 +1905,24 @@ static inline size_t folio_size(struct folio *folio)
 	return PAGE_SIZE << folio_order(folio);
 }
 
+/**
+ * folio_estimated_sharers - Estimate the number of sharers of a folio.
+ * @folio: The folio.
+ *
+ * folio_estimated_sharers() aims to serve as a function to efficiently
+ * estimate the number of processes sharing a folio. This is done by
+ * looking at the precise mapcount of the first subpage in the folio, and
+ * assuming the other subpages are the same. This may not be true for large
+ * folios. If you want exact mapcounts for exact calculations, look at
+ * page_mapcount() or folio_total_mapcount().
+ *
+ * Return: The estimated number of processes sharing a folio.
+ */
+static inline int folio_estimated_sharers(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