mm: add tail private fields to struct folio

Because THP_SWAP uses page->private for each page, we must not use the
space which overlaps that field for anything which would conflict with
that. We avoid the conflict on 32-bit systems by disallowing THP_SWAP on
32-bit.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Sidhartha Kumar <[e-mail]>
Cc: Yanteng Si <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/mm_types.h

···
 			atomic_t _pincount;
 #ifdef CONFIG_64BIT
 			unsigned int _folio_nr_pages;
-#endif
+			/* 4 byte gap here */
 	/* private: the union with struct page is transitional */
+			/* Fix THP_SWAP to not use tail->private */
+			unsigned long _private_1;
+#endif
 		};
 		struct page __page_1;
 	};
···
 	/* public: */
 			struct list_head _deferred_list;
 	/* private: the union with struct page is transitional */
+			unsigned long _avail_2a;
+			/* Fix THP_SWAP to not use tail->private */
+			unsigned long _private_2a;
 		};
 		struct page __page_2;
 	};
···
 			offsetof(struct page, pg) + sizeof(struct page))
 FOLIO_MATCH(flags, _flags_1);
 FOLIO_MATCH(compound_head, _head_1);
+#ifdef CONFIG_64BIT
+FOLIO_MATCH(private, _private_1);
+#endif
 #undef FOLIO_MATCH
 #define FOLIO_MATCH(pg, fl)						\
 	static_assert(offsetof(struct folio, fl) ==			\
 			offsetof(struct page, pg) + 2 * sizeof(struct page))
 FOLIO_MATCH(flags, _flags_2);
 FOLIO_MATCH(compound_head, _head_2);
+FOLIO_MATCH(flags, _flags_2a);
+FOLIO_MATCH(compound_head, _head_2a);
+FOLIO_MATCH(private, _private_2a);
 #undef FOLIO_MATCH
 
 /**