ovl_set_impure
err = ovl_set_impure(c->parent, upperdir);
err = ovl_set_impure(c->parent, c->destdir);
err = ovl_set_impure(new->d_parent, new_upperdir);
err = ovl_set_impure(old->d_parent, old_upperdir);
err = ovl_set_impure(parent, ovl_dentry_upper(parent));
err = ovl_set_impure(dentry->d_parent, upper->d_parent);
int ovl_set_impure(struct dentry *dentry, struct dentry *upperdentry);