NFSv4.1: Fix the r/wsize checking

The intention of nfs4_session_set_rwsize() was to cap the r/wsize to the
buffer sizes negotiated by the CREATE_SESSION. The initial code had a
bug whereby we would not check the values negotiated by nfs_probe_fsinfo()
(the assumption being that CREATE_SESSION will always negotiate buffer values
that are sane w.r.t. the server's preferred r/wsizes) but would only check
values set by the user in the 'mount' command.

The code was changed in 4.11 to _always_ set the r/wsize, meaning that we
now never use the server preferred r/wsizes. This is the regression that
this patch fixes.
Also rename the function to nfs4_session_limit_rwsize() in order to avoid
future confusion.

Fixes: 03385332 (NFSv4.1 respect server's max size in CREATE_SESSION")
Cc: stable@vger.kernel.org # v4.11+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs4client.c

@@ -950,10 +950,10 @@ EXPORT_SYMBOL_GPL(nfs4_set_ds_client);
 
 /*
  * Session has been established, and the client marked ready.
- * Set the mount rsize and wsize with negotiated fore channel
- * attributes which will be bound checked in nfs_server_set_fsinfo.
+ * Limit the mount rsize, wsize and dtsize using negotiated fore
+ * channel attributes.
  */
-static void nfs4_session_set_rwsize(struct nfs_server *server)
+static void nfs4_session_limit_rwsize(struct nfs_server *server)
 {
 #ifdef CONFIG_NFS_V4_1
 	struct nfs4_session *sess;
@@ -966,9 +966,11 @@ static void nfs4_session_set_rwsize(struct nfs_server *server)
 	server_resp_sz = sess->fc_attrs.max_resp_sz - nfs41_maxread_overhead;
 	server_rqst_sz = sess->fc_attrs.max_rqst_sz - nfs41_maxwrite_overhead;
 
-	if (!server->rsize || server->rsize > server_resp_sz)
+	if (server->dtsize > server_resp_sz)
+		server->dtsize = server_resp_sz;
+	if (server->rsize > server_resp_sz)
 		server->rsize = server_resp_sz;
-	if (!server->wsize || server->wsize > server_rqst_sz)
+	if (server->wsize > server_rqst_sz)
 		server->wsize = server_rqst_sz;
 #endif /* CONFIG_NFS_V4_1 */
 }
@@ -1015,12 +1017,12 @@ static int nfs4_server_common_setup(struct nfs_server *server,
 			(unsigned long long) server->fsid.minor);
 	nfs_display_fhandle(mntfh, "Pseudo-fs root FH");
 
-	nfs4_session_set_rwsize(server);
-
 	error = nfs_probe_fsinfo(server, mntfh, fattr);
 	if (error < 0)
 		goto out;
 
+	nfs4_session_limit_rwsize(server);
+
 	if (server->namelen == 0 || server->namelen > NFS4_MAXNAMLEN)
 		server->namelen = NFS4_MAXNAMLEN;