net/9p: Fix kernel crash with msize 512K

With msize equal to 512K (PAGE_SIZE * VIRTQUEUE_NUM), we hit multiple
crashes. This patch fix those.
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: NEric Van Hensbergen <ericvh@gmail.com>

net/9p/trans_virtio.c

@@ -263,7 +263,6 @@ p9_virtio_request(struct p9_client *client, struct p9_req_t *req)
 {
 	int in, out, inp, outp;
 	struct virtio_chan *chan = client->trans;
-	char *rdata = (char *)req->rc+sizeof(struct p9_fcall);
 	unsigned long flags;
 	size_t pdata_off = 0;
 	struct trans_rpage_info *rpinfo = NULL;
@@ -346,7 +345,8 @@ p9_virtio_request(struct p9_client *client, struct p9_req_t *req)
 		 * Arrange in such a way that server places header in the
 		 * alloced memory and payload onto the user buffer.
 		 */
-		inp = pack_sg_list(chan->sg, out, VIRTQUEUE_NUM, rdata, 11);
+		inp = pack_sg_list(chan->sg, out,
+				   VIRTQUEUE_NUM, req->rc->sdata, 11);
 		/*
 		 * Running executables in the filesystem may result in
 		 * a read request with kernel buffer as opposed to user buffer.
@@ -366,8 +366,8 @@ p9_virtio_request(struct p9_client *client, struct p9_req_t *req)
 		}
 		in += inp;
 	} else {
-		in = pack_sg_list(chan->sg, out, VIRTQUEUE_NUM, rdata,
-				req->rc->capacity);
+		in = pack_sg_list(chan->sg, out, VIRTQUEUE_NUM,
+				  req->rc->sdata, req->rc->capacity);
 	}
 
 	err = virtqueue_add_buf(chan->vq, chan->sg, out, in, req->tc);
@@ -592,7 +592,14 @@ static struct p9_trans_module p9_virtio_trans = {
 	.close = p9_virtio_close,
 	.request = p9_virtio_request,
 	.cancel = p9_virtio_cancel,
-	.maxsize = PAGE_SIZE*VIRTQUEUE_NUM,
+
+	/*
+	 * We leave one entry for input and one entry for response
+	 * headers. We also skip one more entry to accomodate, address
+	 * that are not at page boundary, that can result in an extra
+	 * page in zero copy.
+	 */
+	.maxsize = PAGE_SIZE * (VIRTQUEUE_NUM - 3),
 	.pref = P9_TRANS_PREF_PAYLOAD_SEP,
 	.def = 0,
 	.owner = THIS_MODULE,