loop: add lo_submit_rw_aio()

Refactor lo_rw_aio() by extracting the I/O submission logic into a new
helper function lo_submit_rw_aio(). This further improves code organization
by separating the I/O preparation, submission, and completion handling into
distinct phases.

Prepare for using NOWAIT to improve loop performance.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

drivers/block/loop.c

@@ -393,38 +393,32 @@ static int lo_rw_aio_prep(struct loop_device *lo, struct loop_cmd *cmd,
 	return 0;
 }
 
-static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
-		     loff_t pos, int rw)
+static int lo_submit_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
+			    int nr_bvec, int rw)
 {
-	struct iov_iter iter;
-	struct bio_vec *bvec;
 	struct request *rq = blk_mq_rq_from_pdu(cmd);
-	struct bio *bio = rq->bio;
 	struct file *file = lo->lo_backing_file;
-	unsigned int offset;
-	int nr_bvec = lo_cmd_nr_bvec(cmd);
+	struct iov_iter iter;
 	int ret;
 
-	ret = lo_rw_aio_prep(lo, cmd, nr_bvec, pos);
-	if (unlikely(ret))
-		return ret;
-
 	if (cmd->bvec) {
-		offset = 0;
-		bvec = cmd->bvec;
+		iov_iter_bvec(&iter, rw, cmd->bvec, nr_bvec, blk_rq_bytes(rq));
+		iter.iov_offset = 0;
 	} else {
+		struct bio *bio = rq->bio;
+		struct bio_vec *bvec = __bvec_iter_bvec(bio->bi_io_vec,
+				bio->bi_iter);
+
 		/*
 		 * Same here, this bio may be started from the middle of the
 		 * 'bvec' because of bio splitting, so offset from the bvec
 		 * must be passed to iov iterator
 		 */
-		offset = bio->bi_iter.bi_bvec_done;
-		bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
+		iov_iter_bvec(&iter, rw, bvec, nr_bvec, blk_rq_bytes(rq));
+		iter.iov_offset = bio->bi_iter.bi_bvec_done;
 	}
 	atomic_set(&cmd->ref, 2);
 
-	iov_iter_bvec(&iter, rw, bvec, nr_bvec, blk_rq_bytes(rq));
-	iter.iov_offset = offset;
 
 	if (rw == ITER_SOURCE) {
 		kiocb_start_write(&cmd->iocb);
@@ -433,7 +427,20 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 		ret = file->f_op->read_iter(&cmd->iocb, &iter);
 
 	lo_rw_aio_do_completion(cmd);
+	return ret;
+}
+
+static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
+		     loff_t pos, int rw)
+{
+	int nr_bvec = lo_cmd_nr_bvec(cmd);
+	int ret;
+
+	ret = lo_rw_aio_prep(lo, cmd, nr_bvec, pos);
+	if (unlikely(ret))
+		return ret;
 
+	ret = lo_submit_rw_aio(lo, cmd, nr_bvec, rw);
 	if (ret != -EIOCBQUEUED)
 		lo_rw_aio_complete(&cmd->iocb, ret);
 	return -EIOCBQUEUED;