io_uring: better locality for rsrc fields

ring has two types of resource-related fields: used for request
submission, and field needed for update/registration. Reshuffle them
into these two groups for better locality and readability. The second
group is not in the hot path, so it's natural to place them somewhere in
the end. Also update an outdated comment.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/05b34795bb4440f4ec4510f08abd5a31830f8ca0.1621201931.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -391,21 +391,17 @@ struct io_ring_ctx {
 	struct list_head	sqd_list;
 
 	/*
-	 * If used, fixed file set. Writers must ensure that ->refs is dead,
-	 * readers must ensure that ->refs is alive as long as the file* is
-	 * used. Only updated through io_uring_register(2).
+	 * Fixed resources fast path, should be accessed only under uring_lock,
+	 * and updated through io_uring_register(2)
 	 */
-	struct io_rsrc_data	*file_data;
+	struct io_rsrc_node	*rsrc_node;
+
 	struct io_file_table	file_table;
 	unsigned		nr_user_files;
-
-	/* if used, fixed mapped user buffers */
-	struct io_rsrc_data	*buf_data;
 	unsigned		nr_user_bufs;
 	struct io_mapped_ubuf	**user_bufs;
 
 	struct xarray		io_buffers;
-
 	struct xarray		personalities;
 	u32			pers_next;
 
@@ -437,16 +433,21 @@ struct io_ring_ctx {
 		bool			poll_multi_file;
 	} ____cacheline_aligned_in_smp;
 
-	struct delayed_work		rsrc_put_work;
-	struct llist_head		rsrc_put_llist;
-	struct list_head		rsrc_ref_list;
-	spinlock_t			rsrc_ref_lock;
-	struct io_rsrc_node		*rsrc_node;
-	struct io_rsrc_node		*rsrc_backup_node;
-	struct io_mapped_ubuf		*dummy_ubuf;
-
 	struct io_restriction		restrictions;
 
+	/* slow path rsrc auxilary data, used by update/register */
+	struct {
+		struct io_rsrc_node		*rsrc_backup_node;
+		struct io_mapped_ubuf		*dummy_ubuf;
+		struct io_rsrc_data		*file_data;
+		struct io_rsrc_data		*buf_data;
+
+		struct delayed_work		rsrc_put_work;
+		struct llist_head		rsrc_put_llist;
+		struct list_head		rsrc_ref_list;
+		spinlock_t			rsrc_ref_lock;
+	};
+
 	/* Keep this last, we don't need it for the fast path */
 	struct {
 		#if defined(CONFIG_UNIX)