xprtrdma: Provide a buffer to pad Write chunks of unaligned length

This is a buffer to be left persistently registered while a
connection is up. Connection tear-down will automatically DMA-unmap,
invalidate, and dereg the MR. A persistently registered buffer is
lower in cost to provide, and it can never be coalesced into the
RDMA segment that carries the data payload.

An RPC that provisions a Write chunk with a non-aligned length now
uses this MR rather than the tail buffer of the RPC's rq_rcv_buf.

Reviewed-By: Tom Talpey <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

include/trace/events/rpcrdma.h

···
 
 	TP_fast_assign(
 		const struct rpcrdma_req *req = mr->mr_req;
-		const struct rpc_task *task = req->rl_slot.rq_task;
 
-		__entry->task_id = task->tk_pid;
-		__entry->client_id = task->tk_client->cl_clid;
+		if (req) {
+			const struct rpc_task *task = req->rl_slot.rq_task;
+
+			__entry->task_id = task->tk_pid;
+			__entry->client_id = task->tk_client->cl_clid;
+		} else {
+			__entry->task_id = 0;
+			__entry->client_id = -1;
+		}
 		__entry->mr_id  = mr->mr_ibmr->res.id;
 		__entry->nents  = mr->mr_nents;
 		__entry->handle = mr->mr_handle;
···
 DEFINE_RDCH_EVENT(read);
 DEFINE_WRCH_EVENT(write);
 DEFINE_WRCH_EVENT(reply);
+DEFINE_WRCH_EVENT(wp);
 
 TRACE_DEFINE_ENUM(rpcrdma_noch);
 TRACE_DEFINE_ENUM(rpcrdma_noch_pullup);

net/sunrpc/xprtrdma/frwr_ops.c

···
 	 */
 	rpcrdma_force_disconnect(ep);
 }
+
+/**
+ * frwr_wp_create - Create an MR for padding Write chunks
+ * @r_xprt: transport resources to use
+ *
+ * Return 0 on success, negative errno on failure.
+ */
+int frwr_wp_create(struct rpcrdma_xprt *r_xprt)
+{
+	struct rpcrdma_ep *ep = r_xprt->rx_ep;
+	struct rpcrdma_mr_seg seg;
+	struct rpcrdma_mr *mr;
+
+	mr = rpcrdma_mr_get(r_xprt);
+	if (!mr)
+		return -EAGAIN;
+	mr->mr_req = NULL;
+	ep->re_write_pad_mr = mr;
+
+	seg.mr_len = XDR_UNIT;
+	seg.mr_page = virt_to_page(ep->re_write_pad);
+	seg.mr_offset = offset_in_page(ep->re_write_pad);
+	if (IS_ERR(frwr_map(r_xprt, &seg, 1, true, xdr_zero, mr)))
+		return -EIO;
+	trace_xprtrdma_mr_fastreg(mr);
+
+	mr->mr_cqe.done = frwr_wc_fastreg;
+	mr->mr_regwr.wr.next = NULL;
+	mr->mr_regwr.wr.wr_cqe = &mr->mr_cqe;
+	mr->mr_regwr.wr.num_sge = 0;
+	mr->mr_regwr.wr.opcode = IB_WR_REG_MR;
+	mr->mr_regwr.wr.send_flags = 0;
+
+	return ib_post_send(ep->re_id->qp, &mr->mr_regwr.wr, NULL);
+}

net/sunrpc/xprtrdma/rpc_rdma.c

···
 		page_base = 0;
 	}
 
-	if (type == rpcrdma_readch)
-		goto out;
-
-	/* When encoding a Write chunk, some servers need to see an
-	 * extra segment for non-XDR-aligned Write chunks. The upper
-	 * layer provides space in the tail iovec that may be used
-	 * for this purpose.
-	 */
-	if (type == rpcrdma_writech && r_xprt->rx_ep->re_implicit_roundup)
+	if (type == rpcrdma_readch || type == rpcrdma_writech)
 		goto out;
 
 	if (xdrbuf->tail[0].iov_len)
···
 				     enum rpcrdma_chunktype wtype)
 {
 	struct xdr_stream *xdr = &req->rl_stream;
+	struct rpcrdma_ep *ep = r_xprt->rx_ep;
 	struct rpcrdma_mr_seg *seg;
 	struct rpcrdma_mr *mr;
 	int nsegs, nchunks;
···
 		nchunks++;
 		nsegs -= mr->mr_nents;
 	} while (nsegs);
+
+	if (xdr_pad_size(rqst->rq_rcv_buf.page_len)) {
+		if (encode_rdma_segment(xdr, ep->re_write_pad_mr) < 0)
+			return -EMSGSIZE;
+
+		trace_xprtrdma_chunk_wp(rqst->rq_task, ep->re_write_pad_mr,
+					nsegs);
+		r_xprt->rx_stats.write_chunk_count++;
+		r_xprt->rx_stats.total_rdma_request += mr->mr_length;
+		nchunks++;
+		nsegs -= mr->mr_nents;
+	}
 
 	/* Update count of segments in this Write chunk */
 	*segcount = cpu_to_be32(nchunks);

net/sunrpc/xprtrdma/verbs.c

···
 		goto out;
 	}
 	rpcrdma_mrs_create(r_xprt);
+	frwr_wp_create(r_xprt);
 
 out:
 	trace_xprtrdma_connect(r_xprt, rc);

net/sunrpc/xprtrdma/xprt_rdma.h

···
 /*
  * RDMA Endpoint -- connection endpoint details
  */
+struct rpcrdma_mr;
 struct rpcrdma_ep {
 	struct kref		re_kref;
 	struct rdma_cm_id 	*re_id;
 	struct ib_pd		*re_pd;
 	unsigned int		re_max_rdma_segs;
 	unsigned int		re_max_fr_depth;
+	struct rpcrdma_mr	*re_write_pad_mr;
 	bool			re_implicit_roundup;
 	enum ib_mr_type		re_mrtype;
 	struct completion	re_done;
···
 	unsigned int		re_inline_recv;	/* negotiated */
 
 	atomic_t		re_completion_ids;
+
+	char			re_write_pad[XDR_UNIT];
 };
 
 /* Pre-allocate extra Work Requests for handling reverse-direction
···
 void frwr_reminv(struct rpcrdma_rep *rep, struct list_head *mrs);
 void frwr_unmap_sync(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req);
 void frwr_unmap_async(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req);
+int frwr_wp_create(struct rpcrdma_xprt *r_xprt);
 
 /*
  * RPC/RDMA protocol calls - xprtrdma/rpc_rdma.c