+	if (FolioWriteback(folio) &&
+	    wait_on_folio_bit_killable(folio, PG_writeback) < 0)
 		return VM_FAULT_RETRY;

This really screams for a proper wait_on_page_writeback_killable helper
rather than hardcoding the PG_* bit in a random file system.  It also
seems to have missed the conversion to a while loop in
wait_on_page_writeback.

Also this patch seems to be different in style to other by not for now
always using page wrappers in the file systems.  Not that I really care
either way, but it seems inconsistent with the rest.

 /*
- * This is exported only for wait_on_page_locked/wait_on_page_writeback, etc.,
+ * This is exported only for wait_on_folio_locked/wait_on_folio_writeback, etc.,
  * and should not be used directly.
  */
-extern void wait_on_page_bit(struct page *page, int bit_nr);
-extern int wait_on_page_bit_killable(struct page *page, int bit_nr);
+extern void wait_on_folio_bit(struct folio *folio, int bit_nr);
+extern int wait_on_folio_bit_killable(struct folio *folio, int bit_nr);

Well, the above code obviously ignored this comment :(  Maybe an
__ prefix is a bit more of hint?