SUNRPC: Prepare for xdr_stream-style decoding on the server-side

A "permanent" struct xdr_stream is allocated in struct svc_rqst so
that it is usable by all server-side decoders. A per-rqst scratch
buffer is also allocated to handle decoding XDR data items that
cross page boundaries.

To demonstrate how it will be used, add the first call site for the
new svcxdr_init_decode() API.

As an additional part of the overall conversion, add symbolic
constants for successful and failed XDR operations. Returning "0" is
overloaded. Sometimes it means something failed, but sometimes it
means success. To make it more clear when XDR decoding functions
succeed or fail, introduce symbolic constants.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfssvc.c

@@ -1020,6 +1020,8 @@ int nfsd_dispatch(struct svc_rqst *rqstp, __be32 *statp)
 	 * (necessary in the NFSv4.0 compound case)
 	 */
 	rqstp->rq_cachetype = proc->pc_cachetype;
+
+	svcxdr_init_decode(rqstp);
 	if (!proc->pc_decode(rqstp, argv->iov_base))
 		goto out_decode_err;
 

include/linux/sunrpc/svc.h

@@ -247,6 +247,8 @@ struct svc_rqst {
 
 	size_t			rq_xprt_hlen;	/* xprt header len */
 	struct xdr_buf		rq_arg;
+	struct xdr_stream	rq_arg_stream;
+	struct page		*rq_scratch_page;
 	struct xdr_buf		rq_res;
 	struct page		*rq_pages[RPCSVC_MAXPAGES + 1];
 	struct page *		*rq_respages;	/* points into rq_pages */
@@ -557,4 +559,18 @@ static inline void svc_reserve_auth(struct svc_rqst *rqstp, int space)
 	svc_reserve(rqstp, space + rqstp->rq_auth_slack);
 }
 
+/**
+ * svcxdr_init_decode - Prepare an xdr_stream for svc Call decoding
+ * @rqstp: controlling server RPC transaction context
+ *
+ */
+static inline void svcxdr_init_decode(struct svc_rqst *rqstp)
+{
+	struct xdr_stream *xdr = &rqstp->rq_arg_stream;
+	struct kvec *argv = rqstp->rq_arg.head;
+
+	xdr_init_decode(xdr, &rqstp->rq_arg, argv->iov_base, NULL);
+	xdr_set_scratch_page(xdr, rqstp->rq_scratch_page);
+}
+
 #endif /* SUNRPC_SVC_H */

net/sunrpc/svc.c

@@ -614,6 +614,10 @@ svc_rqst_alloc(struct svc_serv *serv, struct svc_pool *pool, int node)
 	rqstp->rq_server = serv;
 	rqstp->rq_pool = pool;
 
+	rqstp->rq_scratch_page = alloc_pages_node(node, GFP_KERNEL, 0);
+	if (!rqstp->rq_scratch_page)
+		goto out_enomem;
+
 	rqstp->rq_argp = kmalloc_node(serv->sv_xdrsize, GFP_KERNEL, node);
 	if (!rqstp->rq_argp)
 		goto out_enomem;
@@ -842,6 +846,7 @@ void
 svc_rqst_free(struct svc_rqst *rqstp)
 {
 	svc_release_buffer(rqstp);
+	put_page(rqstp->rq_scratch_page);
 	kfree(rqstp->rq_resp);
 	kfree(rqstp->rq_argp);
 	kfree(rqstp->rq_auth_data);