SUNRPC: Add a transport-specific private field in rpc_rqst

Currently there's a hidden and indirect mechanism for finding the
rpcrdma_req that goes with an rpc_rqst. It depends on getting from
the rq_buffer pointer in struct rpc_rqst to the struct
rpcrdma_regbuf that controls that buffer, and then to the struct
rpcrdma_req it goes with.

This was done back in the day to avoid the need to add a per-rqst
pointer or to alter the buf_free API when support for RPC-over-RDMA
was introduced.

I'm about to change the way regbuf's work to support larger inline
thresholds. Now is a good time to replace this indirect mechanism
with something that is more straightforward. I guess this should be
considered a clean up.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/linux/sunrpc/xprt.h

@@ -83,6 +83,7 @@ struct rpc_rqst {
 	void (*rq_release_snd_buf)(struct rpc_rqst *); /* release rq_enc_pages */
 	struct list_head	rq_list;
 
+	void			*rq_xprtdata;	/* Per-xprt private data */
 	void			*rq_buffer;	/* Call XDR encode buffer */
 	size_t			rq_callsize;
 	void			*rq_rbuffer;	/* Reply XDR decode buffer */

net/sunrpc/xprtrdma/backchannel.c

@@ -55,11 +55,9 @@ static int rpcrdma_bc_setup_rqst(struct rpcrdma_xprt *r_xprt,
 	rb = rpcrdma_alloc_regbuf(ia, size, GFP_KERNEL);
 	if (IS_ERR(rb))
 		goto out_fail;
-	rb->rg_owner = req;
 	req->rl_sendbuf = rb;
-	/* so that rpcr_to_rdmar works when receiving a request */
-	rqst->rq_buffer = (void *)req->rl_sendbuf->rg_base;
-	xdr_buf_init(&rqst->rq_snd_buf, rqst->rq_buffer, size);
+	xdr_buf_init(&rqst->rq_snd_buf, rb->rg_base, size);
+	rpcrdma_set_xprtdata(rqst, req);
 	return 0;
 
 out_fail:

net/sunrpc/xprtrdma/transport.c

@@ -523,6 +523,7 @@ xprt_rdma_allocate(struct rpc_task *task)
 out:
 	dprintk("RPC:       %s: size %zd, request 0x%p\n", __func__, size, req);
 	req->rl_connect_cookie = 0;	/* our reserved value */
+	rpcrdma_set_xprtdata(rqst, req);
 	rqst->rq_buffer = req->rl_sendbuf->rg_base;
 	rqst->rq_rbuffer = (char *)rqst->rq_buffer + rqst->rq_rcvsize;
 	return 0;
@@ -559,7 +560,6 @@ out_sendbuf:
 	rb = rpcrdma_alloc_regbuf(&r_xprt->rx_ia, size, flags);
 	if (IS_ERR(rb))
 		goto out_fail;
-	rb->rg_owner = req;
 
 	r_xprt->rx_stats.hardway_register_count += size;
 	rpcrdma_free_regbuf(&r_xprt->rx_ia, req->rl_sendbuf);

net/sunrpc/xprtrdma/verbs.c

@@ -1210,7 +1210,6 @@ rpcrdma_alloc_regbuf(struct rpcrdma_ia *ia, size_t size, gfp_t flags)
 	iov->length = size;
 	iov->lkey = ia->ri_pd->local_dma_lkey;
 	rb->rg_size = size;
-	rb->rg_owner = NULL;
 	return rb;
 
 out_free:

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -113,7 +113,6 @@ struct rpcrdma_ep {
 
 struct rpcrdma_regbuf {
 	size_t			rg_size;
-	struct rpcrdma_req	*rg_owner;
 	struct ib_sge		rg_iov;
 	__be32			rg_base[0] __attribute__ ((aligned(256)));
 };
@@ -297,14 +296,16 @@ struct rpcrdma_req {
 	struct rpcrdma_mr_seg	rl_segments[RPCRDMA_MAX_SEGS];
 };
 
+static inline void
+rpcrdma_set_xprtdata(struct rpc_rqst *rqst, struct rpcrdma_req *req)
+{
+	rqst->rq_xprtdata = req;
+}
+
 static inline struct rpcrdma_req *
 rpcr_to_rdmar(struct rpc_rqst *rqst)
 {
-	void *buffer = rqst->rq_buffer;
-	struct rpcrdma_regbuf *rb;
-
-	rb = container_of(buffer, struct rpcrdma_regbuf, rg_base);
-	return rb->rg_owner;
+	return rqst->rq_xprtdata;
 }
 
 /*