io_uring: optimise kiocb layout

We want ->comp_list in the second cacheline, which is hotter comparing
to the 3rd. Swap the field with ->link, which is not as hot and
controlled by flags and so not accessed unless there is a link.

By the way add a couple of comments for io_kiocb fields.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9d9dde31f8f62279a5f48c575bbc27b8290edc0c.1633373302.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -863,19 +863,22 @@ struct io_kiocb {
 	struct task_struct		*task;
 	u64				user_data;
 
-	struct io_kiocb			*link;
 	struct percpu_ref		*fixed_rsrc_refs;
 
-	/* used with ctx->iopoll_list with reads/writes */
+	/* used by request caches, completion batching and iopoll */
 	struct io_wq_work_node		comp_list;
+	struct io_kiocb			*link;
 	struct io_task_work		io_task_work;
 	/* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
 	struct hlist_node		hash_node;
+	/* internal polling, see IORING_FEAT_FAST_POLL */
 	struct async_poll		*apoll;
 	/* store used ubuf, so we can prevent reloading */
 	struct io_mapped_ubuf		*imu;
 	struct io_wq_work		work;
+	/* custom credentials, valid IFF REQ_F_CREDS is set */
 	const struct cred		*creds;
+	/* stores selected buf, valid IFF REQ_F_BUFFER_SELECTED is set */
 	struct io_buffer		*kbuf;
 };