drbd: improve usage of MSG_MORE

It seems to improve performance if we allow the "p_data" header in its
own frame (no MSG_MORE), but sendpage all but the last page with MSG_MORE.
This is also in preparation of a later zero copy receive implementation.

Suggested by Eduard.Guzovsky@stratus.com on drbd-dev.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/block/drbd/drbd_main.c

@@ -2272,17 +2272,17 @@ static int we_should_drop_the_connection(struct drbd_conf *mdev, struct socket *
  * with page_count == 0 or PageSlab.
  */
 static int _drbd_no_send_page(struct drbd_conf *mdev, struct page *page,
-		   int offset, size_t size)
+		   int offset, size_t size, unsigned msg_flags)
 {
-	int sent = drbd_send(mdev, mdev->data.socket, kmap(page) + offset, size, 0);
+	int sent = drbd_send(mdev, mdev->data.socket, kmap(page) + offset, size, msg_flags);
 	kunmap(page);
 	if (sent == size)
 		mdev->send_cnt += size>>9;
 	return sent == size;
 }
 
 static int _drbd_send_page(struct drbd_conf *mdev, struct page *page,
-		    int offset, size_t size)
+		    int offset, size_t size, unsigned msg_flags)
 {
 	mm_segment_t oldfs = get_fs();
 	int sent, ok;
@@ -2295,14 +2295,15 @@ static int _drbd_send_page(struct drbd_conf *mdev, struct page *page,
 	 * __page_cache_release a page that would actually still be referenced
 	 * by someone, leading to some obscure delayed Oops somewhere else. */
 	if (disable_sendpage || (page_count(page) < 1) || PageSlab(page))
-		return _drbd_no_send_page(mdev, page, offset, size);
+		return _drbd_no_send_page(mdev, page, offset, size, msg_flags);
 
+	msg_flags |= MSG_NOSIGNAL;
 	drbd_update_congested(mdev);
 	set_fs(KERNEL_DS);
 	do {
 		sent = mdev->data.socket->ops->sendpage(mdev->data.socket, page,
 							offset, len,
-							MSG_NOSIGNAL);
+							msg_flags);
 		if (sent == -EAGAIN) {
 			if (we_should_drop_the_connection(mdev,
 							  mdev->data.socket))
@@ -2331,9 +2332,11 @@ static int _drbd_send_bio(struct drbd_conf *mdev, struct bio *bio)
 {
 	struct bio_vec *bvec;
 	int i;
+	/* hint all but last page with MSG_MORE */
 	__bio_for_each_segment(bvec, bio, i, 0) {
 		if (!_drbd_no_send_page(mdev, bvec->bv_page,
-				     bvec->bv_offset, bvec->bv_len))
+				     bvec->bv_offset, bvec->bv_len,
+				     i == bio->bi_vcnt -1 ? 0 : MSG_MORE))
 			return 0;
 	}
 	return 1;
@@ -2343,22 +2346,25 @@ static int _drbd_send_zc_bio(struct drbd_conf *mdev, struct bio *bio)
 {
 	struct bio_vec *bvec;
 	int i;
+	/* hint all but last page with MSG_MORE */
 	__bio_for_each_segment(bvec, bio, i, 0) {
 		if (!_drbd_send_page(mdev, bvec->bv_page,
-				     bvec->bv_offset, bvec->bv_len))
+				     bvec->bv_offset, bvec->bv_len,
+				     i == bio->bi_vcnt -1 ? 0 : MSG_MORE))
 			return 0;
 	}
-
 	return 1;
 }
 
 static int _drbd_send_zc_ee(struct drbd_conf *mdev, struct drbd_epoch_entry *e)
 {
 	struct page *page = e->pages;
 	unsigned len = e->size;
+	/* hint all but last page with MSG_MORE */
 	page_chain_for_each(page) {
 		unsigned l = min_t(unsigned, len, PAGE_SIZE);
-		if (!_drbd_send_page(mdev, page, 0, l))
+		if (!_drbd_send_page(mdev, page, 0, l,
+				page_chain_next(page) ? MSG_MORE : 0))
 			return 0;
 		len -= l;
 	}
@@ -2438,11 +2444,11 @@ int drbd_send_dblock(struct drbd_conf *mdev, struct drbd_request *req)
 	p.dp_flags = cpu_to_be32(dp_flags);
 	set_bit(UNPLUG_REMOTE, &mdev->flags);
 	ok = (sizeof(p) ==
-		drbd_send(mdev, mdev->data.socket, &p, sizeof(p), MSG_MORE));
+		drbd_send(mdev, mdev->data.socket, &p, sizeof(p), dgs ? MSG_MORE : 0));
 	if (ok && dgs) {
 		dgb = mdev->int_dig_out;
 		drbd_csum_bio(mdev, mdev->integrity_w_tfm, req->master_bio, dgb);
-		ok = drbd_send(mdev, mdev->data.socket, dgb, dgs, MSG_MORE);
+		ok = drbd_send(mdev, mdev->data.socket, dgb, dgs, 0);
 	}
 	if (ok) {
 		if (mdev->net_conf->wire_protocol == DRBD_PROT_A)
@@ -2491,11 +2497,11 @@ int drbd_send_block(struct drbd_conf *mdev, enum drbd_packets cmd,
 		return 0;
 
 	ok = sizeof(p) == drbd_send(mdev, mdev->data.socket, &p,
-					sizeof(p), MSG_MORE);
+					sizeof(p), dgs ? MSG_MORE : 0);
 	if (ok && dgs) {
 		dgb = mdev->int_dig_out;
 		drbd_csum_ee(mdev, mdev->integrity_w_tfm, e, dgb);
-		ok = drbd_send(mdev, mdev->data.socket, dgb, dgs, MSG_MORE);
+		ok = drbd_send(mdev, mdev->data.socket, dgb, dgs, 0);
 	}
 	if (ok)
 		ok = _drbd_send_zc_ee(mdev, e);