NFSD: Ensure that xdr_write_pages updates rq_next_page

All other NFSv[23] procedures manage to keep page_ptr and
rq_next_page in lock step.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs3xdr.c

@@ -828,7 +828,8 @@ nfs3svc_encode_readlinkres(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 			return false;
 		if (xdr_stream_encode_u32(xdr, resp->len) < 0)
 			return false;
-		xdr_write_pages(xdr, resp->pages, 0, resp->len);
+		svcxdr_encode_opaque_pages(rqstp, xdr, resp->pages, 0,
+					   resp->len);
 		if (svc_encode_result_payload(rqstp, head->iov_len, resp->len) < 0)
 			return false;
 		break;
@@ -859,8 +860,9 @@ nfs3svc_encode_readres(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 			return false;
 		if (xdr_stream_encode_u32(xdr, resp->count) < 0)
 			return false;
-		xdr_write_pages(xdr, resp->pages, rqstp->rq_res.page_base,
-				resp->count);
+		svcxdr_encode_opaque_pages(rqstp, xdr, resp->pages,
+					   rqstp->rq_res.page_base,
+					   resp->count);
 		if (svc_encode_result_payload(rqstp, head->iov_len, resp->count) < 0)
 			return false;
 		break;
@@ -961,7 +963,8 @@ nfs3svc_encode_readdirres(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 			return false;
 		if (!svcxdr_encode_cookieverf3(xdr, resp->verf))
 			return false;
-		xdr_write_pages(xdr, dirlist->pages, 0, dirlist->len);
+		svcxdr_encode_opaque_pages(rqstp, xdr, dirlist->pages, 0,
+					   dirlist->len);
 		/* no more entries */
 		if (xdr_stream_encode_item_absent(xdr) < 0)
 			return false;

fs/nfsd/nfsxdr.c

@@ -468,7 +468,8 @@ nfssvc_encode_readlinkres(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 	case nfs_ok:
 		if (xdr_stream_encode_u32(xdr, resp->len) < 0)
 			return false;
-		xdr_write_pages(xdr, &resp->page, 0, resp->len);
+		svcxdr_encode_opaque_pages(rqstp, xdr, &resp->page, 0,
+					   resp->len);
 		if (svc_encode_result_payload(rqstp, head->iov_len, resp->len) < 0)
 			return false;
 		break;
@@ -491,8 +492,9 @@ nfssvc_encode_readres(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 			return false;
 		if (xdr_stream_encode_u32(xdr, resp->count) < 0)
 			return false;
-		xdr_write_pages(xdr, resp->pages, rqstp->rq_res.page_base,
-				resp->count);
+		svcxdr_encode_opaque_pages(rqstp, xdr, resp->pages,
+					   rqstp->rq_res.page_base,
+					   resp->count);
 		if (svc_encode_result_payload(rqstp, head->iov_len, resp->count) < 0)
 			return false;
 		break;
@@ -511,7 +513,8 @@ nfssvc_encode_readdirres(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 		return false;
 	switch (resp->status) {
 	case nfs_ok:
-		xdr_write_pages(xdr, dirlist->pages, 0, dirlist->len);
+		svcxdr_encode_opaque_pages(rqstp, xdr, dirlist->pages, 0,
+					   dirlist->len);
 		/* no more entries */
 		if (xdr_stream_encode_item_absent(xdr) < 0)
 			return false;

include/linux/sunrpc/svc.h

@@ -508,6 +508,27 @@ static inline void svcxdr_init_encode(struct svc_rqst *rqstp)
 	xdr->rqst = NULL;
 }
 
+/**
+ * svcxdr_encode_opaque_pages - Insert pages into an xdr_stream
+ * @xdr: xdr_stream to be updated
+ * @pages: array of pages to insert
+ * @base: starting offset of first data byte in @pages
+ * @len: number of data bytes in @pages to insert
+ *
+ * After the @pages are added, the tail iovec is instantiated pointing
+ * to end of the head buffer, and the stream is set up to encode
+ * subsequent items into the tail.
+ */
+static inline void svcxdr_encode_opaque_pages(struct svc_rqst *rqstp,
+					      struct xdr_stream *xdr,
+					      struct page **pages,
+					      unsigned int base,
+					      unsigned int len)
+{
+	xdr_write_pages(xdr, pages, base, len);
+	xdr->page_ptr = rqstp->rq_next_page - 1;
+}
+
 /**
  * svcxdr_set_auth_slack -
  * @rqstp: RPC transaction