xprtrdma: Micro-optimize MR DMA-unmapping

Now that rpcrdma_ep is no longer part of rpcrdma_xprt, there are
four or five serial address dereferences needed to get to the
IB device needed for DMA unmapping.

Instead, let's use the same pattern that regbufs use: cache a
pointer to the device in the MR, and use that as the indication
that unmapping is necessary.

This also guarantees that the exact same device is used for DMA
mapping and unmapping, even if the r_xprt's ep has been replaced. I
don't think this can happen today, but future changes might break
this assumption.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/frwr_ops.c

@@ -67,11 +67,11 @@ void frwr_release_mr(struct rpcrdma_mr *mr)
 
 static void frwr_mr_unmap(struct rpcrdma_xprt *r_xprt, struct rpcrdma_mr *mr)
 {
-	if (mr->mr_dir != DMA_NONE) {
+	if (mr->mr_device) {
 		trace_xprtrdma_mr_unmap(mr);
-		ib_dma_unmap_sg(r_xprt->rx_ep->re_id->device,
-				mr->mr_sg, mr->mr_nents, mr->mr_dir);
-		mr->mr_dir = DMA_NONE;
+		ib_dma_unmap_sg(mr->mr_device, mr->mr_sg, mr->mr_nents,
+				mr->mr_dir);
+		mr->mr_device = NULL;
 	}
 }
 
@@ -145,7 +145,7 @@ int frwr_mr_init(struct rpcrdma_xprt *r_xprt, struct rpcrdma_mr *mr)
 
 	mr->mr_xprt = r_xprt;
 	mr->frwr.fr_mr = frmr;
-	mr->mr_dir = DMA_NONE;
+	mr->mr_device = NULL;
 	INIT_LIST_HEAD(&mr->mr_list);
 	init_completion(&mr->frwr.fr_linv_done);
 
@@ -330,6 +330,7 @@ struct rpcrdma_mr_seg *frwr_map(struct rpcrdma_xprt *r_xprt,
 				  mr->mr_dir);
 	if (!dma_nents)
 		goto out_dmamap_err;
+	mr->mr_device = ep->re_id->device;
 
 	ibmr = mr->frwr.fr_mr;
 	n = ib_map_mr_sg(ibmr, mr->mr_sg, dma_nents, NULL, PAGE_SIZE);
@@ -356,7 +357,6 @@ struct rpcrdma_mr_seg *frwr_map(struct rpcrdma_xprt *r_xprt,
 	return seg;
 
 out_dmamap_err:
-	mr->mr_dir = DMA_NONE;
 	trace_xprtrdma_frwr_sgerr(mr, i);
 	return ERR_PTR(-EIO);
 

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -243,6 +243,7 @@ struct rpcrdma_req;
 struct rpcrdma_mr {
 	struct list_head	mr_list;
 	struct rpcrdma_req	*mr_req;
+	struct ib_device	*mr_device;
 	struct scatterlist	*mr_sg;
 	int			mr_nents;
 	enum dma_data_direction	mr_dir;