uvm_rb_maxgap
static vsize_t uvm_rb_maxgap(const struct vm_map_entry *);
entry->maxgap = uvm_rb_maxgap(entry);
brother->maxgap = uvm_rb_maxgap(brother);
if (tmp->maxgap != uvm_rb_maxgap(tmp)) {
(ulong)uvm_rb_maxgap(tmp));