nbd: only set MSG_MORE when we have more to send

commit d61b7f97 upstream.

A user noticed that write performance was horrible over loopback and we
traced it to an inversion of when we need to set MSG_MORE.  It should be
set when we have more bvec's to send, not when we are on the last bvec.
This patch made the test go from 20 iops to 78k iops.
Signed-off-by: Josef Bacik <jbacik@fb.com>
Fixes: 429a787b ("nbd: fix use-after-free of rq/bio in the xmit path")
Signed-off-by: Jens Axboe <axboe@fb.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/block/nbd.c

@@ -239,7 +239,7 @@ static inline int sock_send_bvec(struct nbd_device *nbd, struct bio_vec *bvec,
 /* always call with the tx_lock held */
 static int nbd_send_req(struct nbd_device *nbd, struct request *req)
 {
-	int result, flags;
+	int result;
 	struct nbd_request request;
 	unsigned long size = blk_rq_bytes(req);
 	struct bio *bio;
@@ -270,7 +270,6 @@ static int nbd_send_req(struct nbd_device *nbd, struct request *req)
 	if (nbd_cmd(req) != NBD_CMD_WRITE)
 		return 0;
 
-	flags = 0;
 	bio = req->bio;
 	while (bio) {
 		struct bio *next = bio->bi_next;
@@ -279,9 +278,8 @@ static int nbd_send_req(struct nbd_device *nbd, struct request *req)
 
 		bio_for_each_segment(bvec, bio, iter) {
 			bool is_last = !next && bio_iter_last(bvec, iter);
+			int flags = is_last ? 0 : MSG_MORE;
 
-			if (is_last)
-				flags = MSG_MORE;
 			dprintk(DBG_TX, "%s: request %p: sending %d bytes data\n",
 					nbd->disk->disk_name, req, bvec.bv_len);
 			result = sock_send_bvec(nbd, &bvec, flags);