io_uring: conditional ->async_data allocation

There are opcodes that need ->async_data only in some cases and
allocation it unconditionally may hurt performance. Add an option to
opdef to make move the allocation part from the core io_uring to opcode
specific code.
Note, we can't just set opdef->async_size to zero because there are
other helpers that rely on it, e.g. io_alloc_async_data().
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9dc62be9e88dd0ed63c48365340e8922d2498293.1661342812.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -1450,9 +1450,10 @@ int io_req_prep_async(struct io_kiocb *req)
 		return 0;
 	if (WARN_ON_ONCE(req_has_async_data(req)))
 		return -EFAULT;
-	if (io_alloc_async_data(req))
-		return -EAGAIN;
-
+	if (!io_op_defs[req->opcode].manual_alloc) {
+		if (io_alloc_async_data(req))
+			return -EAGAIN;
+	}
 	return def->prep_async(req);
 }
 

io_uring/opdef.h

@@ -25,6 +25,8 @@ struct io_op_def {
 	unsigned		ioprio : 1;
 	/* supports iopoll */
 	unsigned		iopoll : 1;
+	/* opcode specific path will handle ->async_data allocation if needed */
+	unsigned		manual_alloc : 1;
 	/* size of async data needed, if any */
 	unsigned short		async_size;