[PATCH 1/2] mm: Add folio_put_refs()

[Matthew Wilcox (Oracle)]

This is like folio_put(), but puts N references at once instead of
just one.  It's like put_page_refs(), but does one atomic operation
instead of two, and is available to more than just gup.c.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Reviewed-by: William Kucharski <william.kucharski@xxxxxxxxxx>
---
 include/linux/mm.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c768a7c81b0b..cb98f75b245e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1244,6 +1244,26 @@ static inline void folio_put(struct folio *folio)
 		__put_page(&folio->page);
 }
 
+/**
+ * folio_put_refs - Reduce the reference count on a folio.
+ * @folio: The folio.
+ * @refs: The amount to subtract from the folio's reference count.
+ *
+ * If the folio's reference count reaches zero, the memory will be
+ * released back to the page allocator and may be used by another
+ * allocation immediately.  Do not access the memory or the struct folio
+ * after calling folio_put_refs() unless you can be sure that these weren't
+ * the last references.
+ *
+ * Context: May be called in process or interrupt context, but not in NMI
+ * context.  May be called while holding a spinlock.
+ */
+static inline void folio_put_refs(struct folio *folio, int refs)
+{
+	if (folio_ref_sub_and_test(folio, refs))
+		__put_page(&folio->page);
+}
+
 static inline void put_page(struct page *page)
 {
 	struct folio *folio = page_folio(page);
-- 
2.33.0