NFS: report how long an NFS file system has been mounted

Add a field in nfs_server to record a timestamp when a mount succeeds.
Report the number of seconds the file system has been mounted via
nfs_show_stats().

Test plan:
Mount an NFS file system, watch the mountstats reports and compare with
clock time.
Signed-off-by: NChuck Lever <cel@netapp.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/inode.c

@@ -299,6 +299,9 @@ nfs_sb_init(struct super_block *sb, rpc_authflavor_t authflavor)
 		goto out_no_root;
 	}
 
+	/* mount time stamp, in seconds */
+	server->mount_time = jiffies;
+
 	/* Get some general file system info */
 	if (server->namelen == 0 &&
 	    server->rpc_ops->pathconf(server, &server->fh, &pathinfo) >= 0)
@@ -674,6 +677,8 @@ static int nfs_show_stats(struct seq_file *m, struct vfsmount *mnt)
 	seq_puts(m, mnt->mnt_sb->s_flags & MS_NODIRATIME ? ",nodiratime" : "");
 	nfs_show_mount_options(m, nfss, 1);
 
+	seq_printf(m, "\n\tage:\t%lu", (jiffies - nfss->mount_time) / HZ);
+
 	seq_printf(m, "\n\tcaps:\t");
 	seq_printf(m, "caps=0x%x", nfss->caps);
 	seq_printf(m, ",wtmult=%d", nfss->wtmult);

include/linux/nfs_fs_sb.h

@@ -35,6 +35,7 @@ struct nfs_server {
 	char *			hostname;	/* remote hostname */
 	struct nfs_fh		fh;
 	struct sockaddr_in	addr;
+	unsigned long		mount_time;	/* when this fs was mounted */
 #ifdef CONFIG_NFS_V4
 	/* Our own IP address, as a null-terminated string.
 	 * This is used to generate the clientid, and the callback address.