mm: add the first tail page to struct folio

Some of the static checkers get confused by extracting the page from the
folio and referring to fields in the first tail page.  Adding these fields
to struct folio lets us avoid doing that.  It has the risk that people
will refer to those fields without checking that the folio is actually a
large folio, so prefix them with underscores and document the preferred
function to use instead.

Link: https://lkml.kernel.org/r/20220902194653.1739778-3-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm_types.h

@@ -245,6 +245,13 @@ struct page {
  * @_refcount: Do not access this member directly.  Use folio_ref_count()
  *    to find how many references there are to this folio.
  * @memcg_data: Memory Control Group data.
+ * @_flags_1: For large folios, additional page flags.
+ * @__head: Points to the folio.  Do not use.
+ * @_folio_dtor: Which destructor to use for this folio.
+ * @_folio_order: Do not use directly, call folio_order().
+ * @_total_mapcount: Do not use directly, call folio_entire_mapcount().
+ * @_pincount: Do not use directly, call folio_maybe_dma_pinned().
+ * @_folio_nr_pages: Do not use directly, call folio_nr_pages().
  *
  * A folio is a physically, virtually and logically contiguous set
  * of bytes.  It is a power-of-two in size, and it is aligned to that
@@ -283,9 +290,17 @@ struct folio {
 		};
 		struct page page;
 	};
+	unsigned long _flags_1;
+	unsigned long __head;
+	unsigned char _folio_dtor;
+	unsigned char _folio_order;
+	atomic_t _total_mapcount;
+	atomic_t _pincount;
+#ifdef CONFIG_64BIT
+	unsigned int _folio_nr_pages;
+#endif
 };
 
-static_assert(sizeof(struct page) == sizeof(struct folio));
 #define FOLIO_MATCH(pg, fl)						\
 	static_assert(offsetof(struct page, pg) == offsetof(struct folio, fl))
 FOLIO_MATCH(flags, flags);
@@ -300,6 +315,19 @@ FOLIO_MATCH(_refcount, _refcount);
 FOLIO_MATCH(memcg_data, memcg_data);
 #endif
 #undef FOLIO_MATCH
+#define FOLIO_MATCH(pg, fl)						\
+	static_assert(offsetof(struct folio, fl) ==			\
+			offsetof(struct page, pg) + sizeof(struct page))
+FOLIO_MATCH(flags, _flags_1);
+FOLIO_MATCH(compound_head, __head);
+FOLIO_MATCH(compound_dtor, _folio_dtor);
+FOLIO_MATCH(compound_order, _folio_order);
+FOLIO_MATCH(compound_mapcount, _total_mapcount);
+FOLIO_MATCH(compound_pincount, _pincount);
+#ifdef CONFIG_64BIT
+FOLIO_MATCH(compound_nr, _folio_nr_pages);
+#endif
+#undef FOLIO_MATCH
 
 static inline atomic_t *folio_mapcount_ptr(struct folio *folio)
 {