pagevec: Add folio_batch

The folio_batch is the same as the pagevec, except that it is typed
to contain folios and not pages.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>

include/linux/pagevec.h

@@ -15,8 +15,10 @@
 #define PAGEVEC_SIZE	15
 
 struct page;
+struct folio;
 struct address_space;
 
+/* Layout must match folio_batch */
 struct pagevec {
 	unsigned char nr;
 	bool percpu_pvec_drained;
@@ -81,4 +83,71 @@ static inline void pagevec_release(struct pagevec *pvec)
 		__pagevec_release(pvec);
 }
 
+/**
+ * struct folio_batch - A collection of folios.
+ *
+ * The folio_batch is used to amortise the cost of retrieving and
+ * operating on a set of folios.  The order of folios in the batch may be
+ * significant (eg delete_from_page_cache_batch()).  Some users of the
+ * folio_batch store "exceptional" entries in it which can be removed
+ * by calling folio_batch_remove_exceptionals().
+ */
+struct folio_batch {
+	unsigned char nr;
+	bool percpu_pvec_drained;
+	struct folio *folios[PAGEVEC_SIZE];
+};
+
+/* Layout must match pagevec */
+static_assert(sizeof(struct pagevec) == sizeof(struct folio_batch));
+static_assert(offsetof(struct pagevec, pages) ==
+		offsetof(struct folio_batch, folios));
+
+/**
+ * folio_batch_init() - Initialise a batch of folios
+ * @fbatch: The folio batch.
+ *
+ * A freshly initialised folio_batch contains zero folios.
+ */
+static inline void folio_batch_init(struct folio_batch *fbatch)
+{
+	fbatch->nr = 0;
+}
+
+static inline unsigned int folio_batch_count(struct folio_batch *fbatch)
+{
+	return fbatch->nr;
+}
+
+static inline unsigned int fbatch_space(struct folio_batch *fbatch)
+{
+	return PAGEVEC_SIZE - fbatch->nr;
+}
+
+/**
+ * folio_batch_add() - Add a folio to a batch.
+ * @fbatch: The folio batch.
+ * @folio: The folio to add.
+ *
+ * The folio is added to the end of the batch.
+ * The batch must have previously been initialised using folio_batch_init().
+ *
+ * Return: The number of slots still available.
+ */
+static inline unsigned folio_batch_add(struct folio_batch *fbatch,
+		struct folio *folio)
+{
+	fbatch->folios[fbatch->nr++] = folio;
+	return fbatch_space(fbatch);
+}
+
+static inline void folio_batch_release(struct folio_batch *fbatch)
+{
+	pagevec_release((struct pagevec *)fbatch);
+}
+
+static inline void folio_batch_remove_exceptionals(struct folio_batch *fbatch)
+{
+	pagevec_remove_exceptionals((struct pagevec *)fbatch);
+}
 #endif /* _LINUX_PAGEVEC_H */