scsi: target: iscsi: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage

Use sendmsg() with MSG_SPLICE_PAGES rather than sendpage.  This allows
multiple pages and multipage folios to be passed through.

TODO: iscsit_fe_sendpage_sg() should perhaps set up a bio_vec array for the
entire set of pages it's going to transfer plus two for the header and
trailer and page fragments to hold the header and trailer - and then call
sendmsg once for the entire message.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Mike Christie <michael.christie@oracle.com>
cc: Maurizio Lombardi <mlombard@redhat.com>
cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
cc: "Martin K. Petersen" <martin.petersen@oracle.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: open-iscsi@googlegroups.com
Link: https://lore.kernel.org/r/20230623225513.2732256-13-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: dhowells

drivers/target/iscsi/iscsi_target_util.c

@@ -1129,6 +1129,8 @@ int iscsit_fe_sendpage_sg(
 	struct iscsit_conn *conn)
 {
 	struct scatterlist *sg = cmd->first_data_sg;
+	struct bio_vec bvec;
+	struct msghdr msghdr = { .msg_flags = MSG_SPLICE_PAGES,	};
 	struct kvec iov;
 	u32 tx_hdr_size, data_len;
 	u32 offset = cmd->first_data_sg_off;
@@ -1172,17 +1174,18 @@ int iscsit_fe_sendpage_sg(
 		u32 space = (sg->length - offset);
 		u32 sub_len = min_t(u32, data_len, space);
 send_pg:
-		tx_sent = conn->sock->ops->sendpage(conn->sock,
-					sg_page(sg), sg->offset + offset, sub_len, 0);
+		bvec_set_page(&bvec, sg_page(sg), sub_len, sg->offset + offset);
+		iov_iter_bvec(&msghdr.msg_iter, ITER_SOURCE, &bvec, 1, sub_len);
+
+		tx_sent = conn->sock->ops->sendmsg(conn->sock, &msghdr,
+						   sub_len);
 		if (tx_sent != sub_len) {
 			if (tx_sent == -EAGAIN) {
-				pr_err("tcp_sendpage() returned"
-						" -EAGAIN\n");
+				pr_err("sendmsg/splice returned -EAGAIN\n");
 				goto send_pg;
 			}
 
-			pr_err("tcp_sendpage() failure: %d\n",
-					tx_sent);
+			pr_err("sendmsg/splice failure: %d\n", tx_sent);
 			return -1;
 		}