xprtrdma: Backchannel can use GFP_KERNEL allocations

The Receive handler runs in process context, thus can use on-demand
GFP_KERNEL allocations instead of pre-allocation.

This makes the xprtrdma backchannel independent of the number of
backchannel session slots provisioned by the Upper Layer protocol.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/backchannel.c

@@ -19,35 +19,6 @@
 
 #undef RPCRDMA_BACKCHANNEL_DEBUG
 
-static int rpcrdma_bc_setup_reqs(struct rpcrdma_xprt *r_xprt,
-				 unsigned int count)
-{
-	struct rpc_xprt *xprt = &r_xprt->rx_xprt;
-	struct rpcrdma_req *req;
-	struct rpc_rqst *rqst;
-	unsigned int i;
-
-	for (i = 0; i < (count << 1); i++) {
-		size_t size;
-
-		size = min_t(size_t, r_xprt->rx_data.inline_rsize, PAGE_SIZE);
-		req = rpcrdma_req_create(r_xprt, size, GFP_KERNEL);
-		if (!req)
-			return -ENOMEM;
-		rqst = &req->rl_slot;
-
-		rqst->rq_xprt = xprt;
-		INIT_LIST_HEAD(&rqst->rq_bc_list);
-		__set_bit(RPC_BC_PA_IN_USE, &rqst->rq_bc_pa_state);
-		spin_lock(&xprt->bc_pa_lock);
-		list_add(&rqst->rq_bc_pa_list, &xprt->bc_pa_list);
-		spin_unlock(&xprt->bc_pa_lock);
-		xdr_buf_init(&rqst->rq_snd_buf, rdmab_data(req->rl_sendbuf),
-			     size);
-	}
-	return 0;
-}
-
 /**
  * xprt_rdma_bc_setup - Pre-allocate resources for handling backchannel requests
  * @xprt: transport associated with these backchannel resources
@@ -58,34 +29,10 @@ static int rpcrdma_bc_setup_reqs(struct rpcrdma_xprt *r_xprt,
 int xprt_rdma_bc_setup(struct rpc_xprt *xprt, unsigned int reqs)
 {
 	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
-	int rc;
-
-	/* The backchannel reply path returns each rpc_rqst to the
-	 * bc_pa_list _after_ the reply is sent. If the server is
-	 * faster than the client, it can send another backward
-	 * direction request before the rpc_rqst is returned to the
-	 * list. The client rejects the request in this case.
-	 *
-	 * 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;
-
-	rc = rpcrdma_bc_setup_reqs(r_xprt, reqs);
-	if (rc)
-		goto out_free;
 
-	r_xprt->rx_buf.rb_bc_srv_max_requests = reqs;
+	r_xprt->rx_buf.rb_bc_srv_max_requests = RPCRDMA_BACKWARD_WRS >> 1;
 	trace_xprtrdma_cb_setup(r_xprt, reqs);
 	return 0;
-
-out_free:
-	xprt_rdma_bc_destroy(xprt, reqs);
-
-out_err:
-	pr_err("RPC:       %s: setup backchannel transport failed\n", __func__);
-	return -ENOMEM;
 }
 
 /**
@@ -213,6 +160,43 @@ void xprt_rdma_bc_free_rqst(struct rpc_rqst *rqst)
 	spin_unlock(&xprt->bc_pa_lock);
 }
 
+static struct rpc_rqst *rpcrdma_bc_rqst_get(struct rpcrdma_xprt *r_xprt)
+{
+	struct rpc_xprt *xprt = &r_xprt->rx_xprt;
+	struct rpcrdma_req *req;
+	struct rpc_rqst *rqst;
+	size_t size;
+
+	spin_lock(&xprt->bc_pa_lock);
+	rqst = list_first_entry_or_null(&xprt->bc_pa_list, struct rpc_rqst,
+					rq_bc_pa_list);
+	if (!rqst)
+		goto create_req;
+	list_del(&rqst->rq_bc_pa_list);
+	spin_unlock(&xprt->bc_pa_lock);
+	return rqst;
+
+create_req:
+	spin_unlock(&xprt->bc_pa_lock);
+
+	/* Set a limit to prevent a remote from overrunning our resources.
+	 */
+	if (xprt->bc_alloc_count >= RPCRDMA_BACKWARD_WRS)
+		return NULL;
+
+	size = min_t(size_t, r_xprt->rx_data.inline_rsize, PAGE_SIZE);
+	req = rpcrdma_req_create(r_xprt, size, GFP_KERNEL);
+	if (!req)
+		return NULL;
+
+	xprt->bc_alloc_count++;
+	rqst = &req->rl_slot;
+	rqst->rq_xprt = xprt;
+	__set_bit(RPC_BC_PA_IN_USE, &rqst->rq_bc_pa_state);
+	xdr_buf_init(&rqst->rq_snd_buf, rdmab_data(req->rl_sendbuf), size);
+	return rqst;
+}
+
 /**
  * rpcrdma_bc_receive_call - Handle a backward direction call
  * @r_xprt: transport receiving the call
@@ -244,18 +228,10 @@ void rpcrdma_bc_receive_call(struct rpcrdma_xprt *r_xprt,
 	pr_info("RPC:       %s: %*ph\n", __func__, size, p);
 #endif
 
-	/* Grab a free bc rqst */
-	spin_lock(&xprt->bc_pa_lock);
-	if (list_empty(&xprt->bc_pa_list)) {
-		spin_unlock(&xprt->bc_pa_lock);
+	rqst = rpcrdma_bc_rqst_get(r_xprt);
+	if (!rqst)
 		goto out_overflow;
-	}
-	rqst = list_first_entry(&xprt->bc_pa_list,
-				struct rpc_rqst, rq_bc_pa_list);
-	list_del(&rqst->rq_bc_pa_list);
-	spin_unlock(&xprt->bc_pa_lock);
 
-	/* Prepare rqst */
 	rqst->rq_reply_bytes_recvd = 0;
 	rqst->rq_xid = *p;