Register .remount_fs handler

Register the missing .remount_fs handler.  This handler isn't strictly
required because the VFS does a pretty good job updating most of the
MS_* flags.  However, there's no harm in using the hook to call the
registered zpl callback for various MS_* flags.  Additionaly, this
allows us to lay the ground work for more complicated argument parsing
in the future.

include/sys/zfs_vfsops.h

@@ -187,6 +187,7 @@ extern int zfs_register_callbacks(zfs_sb_t *zsb);
 extern void zfs_unregister_callbacks(zfs_sb_t *zsb);
 extern int zfs_domount(struct super_block *sb, void *data, int silent);
 extern int zfs_umount(struct super_block *sb);
+extern int zfs_remount(struct super_block *sb, int *flags, char *data);
 extern int zfs_root(zfs_sb_t *zsb, struct inode **ipp);
 extern int zfs_statvfs(struct dentry *dentry, struct kstatfs *statp);
 extern int zfs_vget(struct vfsmount *vfsp, struct inode **ipp, fid_t *fidp);

module/zfs/zfs_vfsops.c

@@ -1291,6 +1291,46 @@ zfs_umount(struct super_block *sb)
 }
 EXPORT_SYMBOL(zfs_umount);
 
+int
+zfs_remount(struct super_block *sb, int *flags, char *data)
+{
+	zfs_sb_t *zsb = sb->s_fs_info;
+	boolean_t readonly = B_FALSE;
+	boolean_t setuid = B_TRUE;
+	boolean_t exec = B_TRUE;
+	boolean_t devices = B_TRUE;
+	boolean_t atime = B_TRUE;
+
+	if (*flags & MS_RDONLY)
+		readonly = B_TRUE;
+
+	if (*flags & MS_NOSUID) {
+		devices = B_FALSE;
+		setuid = B_FALSE;
+	} else {
+		if (*flags & MS_NODEV)
+			devices = B_FALSE;
+	}
+
+	if (*flags & MS_NOEXEC)
+		exec = B_FALSE;
+
+	if (*flags & MS_NOATIME)
+		atime = B_FALSE;
+
+	/*
+	 * Invoke our callbacks to set required flags.
+	 */
+	readonly_changed_cb(zsb, readonly);
+	setuid_changed_cb(zsb, setuid);
+	exec_changed_cb(zsb, exec);
+	devices_changed_cb(zsb, devices);
+	atime_changed_cb(zsb, atime);
+
+	return (0);
+}
+EXPORT_SYMBOL(zfs_remount);
+
 int
 zfs_vget(struct vfsmount *vfsp, struct inode **ipp, fid_t *fidp)
 {

module/zfs/zpl_super.c

@@ -129,6 +129,16 @@ zpl_statfs(struct dentry *dentry, struct kstatfs *statp)
 	return (error);
 }
 
+static int
+zpl_remount_fs(struct super_block *sb, int *flags, char *data)
+{
+	int error;
+	error = -zfs_remount(sb, flags, data);
+	ASSERT3S(error, <=, 0);
+
+	return (error);
+}
+
 static int
 zpl_show_options(struct seq_file *seq, struct vfsmount *vfsp)
 {
@@ -197,7 +207,7 @@ const struct super_operations zpl_super_operations = {
 	.freeze_fs	= NULL,
 	.unfreeze_fs	= NULL,
 	.statfs		= zpl_statfs,
-	.remount_fs	= NULL,
+	.remount_fs	= zpl_remount_fs,
 	.show_options	= zpl_show_options,
 	.show_stats	= NULL,
 };