Index: kern/vfs_mount.c
===================================================================
--- kern/vfs_mount.c
+++ kern/vfs_mount.c
@@ -537,6 +537,7 @@
 	struct vfsopt *opt, *tmp_opt;
 	char *fstype, *fspath, *errmsg;
 	int error, fstypelen, fspathlen, errmsg_len, errmsg_pos;
+	uint64_t fsflag_high;
 
 	errmsg = fspath = NULL;
 	errmsg_len = fspathlen = 0;
@@ -653,6 +654,20 @@
 			fsflags |= MNT_AUTOMOUNTED;
 			vfs_freeopt(optlist, opt);
 		}
+		else if (strcmp(opt->name, "fsflaghigh") == 0) {
+			/*
+			 * This option allows a process doing a mount update
+			 * to set all the high order 32bits of mnt_flag with
+			 * one option.  This is needed, since nmount(2)
+			 * accepts an "int" option and, therefore, only takes
+			 * the low order 32bits.
+			 */
+			if (vfs_copyopt(optlist, "fsflaghigh",
+			    &fsflag_high, sizeof(fsflag_high)) == 0 &&
+			    (fsflag_high & 0xffffffffULL) == 0)
+				fsflags |= fsflag_high;
+			vfs_freeopt(optlist, opt);
+		}
 	}
 
 	/*
Index: usr.sbin/mountd/mountd.c
===================================================================
--- usr.sbin/mountd/mountd.c
+++ usr.sbin/mountd/mountd.c
@@ -1649,6 +1649,7 @@
 	int iovlen;
 	int done;
 	struct nfsex_args eargs;
+	uint64_t fsflag_high;
 
 	if (suspend_nfsd != 0)
 		(void)nfssvc(NFSSVC_SUSPENDNFSD, NULL);
@@ -1706,6 +1707,8 @@
 		build_iovec(&iov, &iovlen, "update", NULL, 0);
 		build_iovec(&iov, &iovlen, "export", &export, sizeof(export));
 		build_iovec(&iov, &iovlen, "errmsg", errmsg, sizeof(errmsg));
+		build_iovec(&iov, &iovlen, "fsflaghigh", &fsflag_high,
+		    sizeof(fsflag_high));
 	}
 
 	for (i = 0; i < num; i++) {
@@ -1736,6 +1739,9 @@
 		iov[3].iov_len = strlen(fsp->f_mntonname) + 1;
 		iov[5].iov_base = fsp->f_mntfromname;
 		iov[5].iov_len = strlen(fsp->f_mntfromname) + 1;
+		/* Use "fsflaghigh" to set the high order bits of f_flags. */
+		fsflag_high = (fsp->f_flags & 0xffffffff00000000ULL);
+
 		errmsg[0] = '\0';
 
 		/*
@@ -2394,6 +2400,7 @@
 	int i, iovlen;
 	int ret;
 	struct nfsex_args nfsea;
+	uint64_t fsflag_high;
 
 	eap = &nfsea.export;
 
@@ -2429,6 +2436,12 @@
 		build_iovec(&iov, &iovlen, "export", eap,
 		    sizeof (struct export_args));
 		build_iovec(&iov, &iovlen, "errmsg", errmsg, sizeof(errmsg));
+
+		/* Use "fsflaghigh" to set the high order bits of f_flags. */
+		fsflag_high = (fsb->f_flags & 0xffffffff00000000ULL);
+		if (fsflag_high != 0)
+			build_iovec(&iov, &iovlen, "fsflaghigh", &fsflag_high,
+			    sizeof(fsflag_high));
 	}
 
 	while (!done) {