Re: [PATCH v4 08/25] mm: Handle per-folio private data

+static inline void attach_page_private(struct page *page, void *data)
+{
+	attach_folio_private((struct folio *)page, data);
+}
+
+static inline void *detach_page_private(struct page *page)
+{
+	return detach_folio_private((struct folio *)page);
+}

I hate these open code casts.  Can't we have a single central
page_to_folio helper, which could also grow a debug check (maybe
under a new config option) to check that it really is called on a
head page?