xprtrdma: Pre-allocate Work Requests for backchannel

Pre-allocate extra send and receive Work Requests needed to handle
backchannel receives and sends.

The transport doesn't know how many extra WRs to pre-allocate until
the xprt_setup_backchannel() call, but that's long after the WRs are
allocated during forechannel setup.

So, use a fixed value for now.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Tested-By: Devesh Sharma <devesh.sharma@avagotech.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/backchannel.c

@@ -125,6 +125,9 @@ int xprt_rdma_bc_setup(struct rpc_xprt *xprt, unsigned int reqs)
 	 * Twice as many rpc_rqsts are prepared to ensure there is
 	 * always an rpc_rqst available as soon as a reply is sent.
 	 */
+	if (reqs > RPCRDMA_BACKWARD_WRS >> 1)
+		goto out_err;
+
 	for (i = 0; i < (reqs << 1); i++) {
 		rqst = kzalloc(sizeof(*rqst), GFP_KERNEL);
 		if (!rqst) {
@@ -161,6 +164,7 @@ int xprt_rdma_bc_setup(struct rpc_xprt *xprt, unsigned int reqs)
 out_free:
 	xprt_rdma_bc_destroy(xprt, reqs);
 
+out_err:
 	pr_err("RPC:       %s: setup backchannel transport failed\n", __func__);
 	return -ENOMEM;
 }

net/sunrpc/xprtrdma/verbs.c

@@ -568,6 +568,7 @@ rpcrdma_ep_create(struct rpcrdma_ep *ep, struct rpcrdma_ia *ia,
 	struct ib_device_attr *devattr = &ia->ri_devattr;
 	struct ib_cq *sendcq, *recvcq;
 	struct ib_cq_init_attr cq_attr = {};
+	unsigned int max_qp_wr;
 	int rc, err;
 
 	if (devattr->max_sge < RPCRDMA_MAX_IOVS) {
@@ -576,18 +577,27 @@ rpcrdma_ep_create(struct rpcrdma_ep *ep, struct rpcrdma_ia *ia,
 		return -ENOMEM;
 	}
 
+	if (devattr->max_qp_wr <= RPCRDMA_BACKWARD_WRS) {
+		dprintk("RPC:       %s: insufficient wqe's available\n",
+			__func__);
+		return -ENOMEM;
+	}
+	max_qp_wr = devattr->max_qp_wr - RPCRDMA_BACKWARD_WRS;
+
 	/* check provider's send/recv wr limits */
-	if (cdata->max_requests > devattr->max_qp_wr)
-		cdata->max_requests = devattr->max_qp_wr;
+	if (cdata->max_requests > max_qp_wr)
+		cdata->max_requests = max_qp_wr;
 
 	ep->rep_attr.event_handler = rpcrdma_qp_async_error_upcall;
 	ep->rep_attr.qp_context = ep;
 	ep->rep_attr.srq = NULL;
 	ep->rep_attr.cap.max_send_wr = cdata->max_requests;
+	ep->rep_attr.cap.max_send_wr += RPCRDMA_BACKWARD_WRS;
 	rc = ia->ri_ops->ro_open(ia, ep, cdata);
 	if (rc)
 		return rc;
 	ep->rep_attr.cap.max_recv_wr = cdata->max_requests;
+	ep->rep_attr.cap.max_recv_wr += RPCRDMA_BACKWARD_WRS;
 	ep->rep_attr.cap.max_send_sge = RPCRDMA_MAX_IOVS;
 	ep->rep_attr.cap.max_recv_sge = 1;
 	ep->rep_attr.cap.max_inline_data = 0;

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -101,6 +101,16 @@ struct rpcrdma_ep {
  */
 #define RPCRDMA_IGNORE_COMPLETION	(0ULL)
 
+/* Pre-allocate extra Work Requests for handling backward receives
+ * and sends. This is a fixed value because the Work Queues are
+ * allocated when the forward channel is set up.
+ */
+#if defined(CONFIG_SUNRPC_BACKCHANNEL)
+#define RPCRDMA_BACKWARD_WRS		(8)
+#else
+#define RPCRDMA_BACKWARD_WRS		(0)
+#endif
+
 /* Registered buffer -- registered kmalloc'd memory for RDMA SEND/RECV
  *
  * The below structure appears at the front of a large region of kmalloc'd