NFSv4.1 provide mount option to toggle trunking discovery

Introduce a new mount option -- trunkdiscovery,notrunkdiscovery -- to
toggle whether or not the client will engage in actively discovery
of trunking locations.

v2 make notrunkdiscovery default
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Fixes: 1976b2b3 ("NFSv4.1 query for fs_location attr on a new file system")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/client.c

@@ -857,7 +857,8 @@ static int nfs_probe_fsinfo(struct nfs_server *server, struct nfs_fh *mntfh, str
 	}
 
 	if (clp->rpc_ops->discover_trunking != NULL &&
-			(server->caps & NFS_CAP_FS_LOCATIONS)) {
+			(server->caps & NFS_CAP_FS_LOCATIONS &&
+			 (server->flags & NFS_MOUNT_TRUNK_DISCOVERY))) {
 		error = clp->rpc_ops->discover_trunking(server, mntfh);
 		if (error < 0)
 			return error;

fs/nfs/fs_context.c

@@ -80,6 +80,7 @@ enum nfs_param {
 	Opt_source,
 	Opt_tcp,
 	Opt_timeo,
+	Opt_trunkdiscovery,
 	Opt_udp,
 	Opt_v,
 	Opt_vers,
@@ -180,6 +181,7 @@ static const struct fs_parameter_spec nfs_fs_parameters[] = {
 	fsparam_string("source",	Opt_source),
 	fsparam_flag  ("tcp",		Opt_tcp),
 	fsparam_u32   ("timeo",		Opt_timeo),
+	fsparam_flag_no("trunkdiscovery", Opt_trunkdiscovery),
 	fsparam_flag  ("udp",		Opt_udp),
 	fsparam_flag  ("v2",		Opt_v),
 	fsparam_flag  ("v3",		Opt_v),
@@ -529,6 +531,12 @@ static int nfs_fs_context_parse_param(struct fs_context *fc,
 		else
 			ctx->flags &= ~NFS_MOUNT_NOCTO;
 		break;
+	case Opt_trunkdiscovery:
+		if (result.negated)
+			ctx->flags &= ~NFS_MOUNT_TRUNK_DISCOVERY;
+		else
+			ctx->flags |= NFS_MOUNT_TRUNK_DISCOVERY;
+		break;
 	case Opt_ac:
 		if (result.negated)
 			ctx->flags |= NFS_MOUNT_NOAC;

include/linux/nfs_fs_sb.h

@@ -151,6 +151,7 @@ struct nfs_server {
 #define NFS_MOUNT_SOFTREVAL		0x800000
 #define NFS_MOUNT_WRITE_EAGER		0x01000000
 #define NFS_MOUNT_WRITE_WAIT		0x02000000
+#define NFS_MOUNT_TRUNK_DISCOVERY	0x04000000
 
 	unsigned int		fattr_valid;	/* Valid attributes */
 	unsigned int		caps;		/* server capabilities */