mr.release(pages);
  public final VM_Address acquire(int pages, MemoryResource mr) {    if (VM.VerifyAssertions) VM._assert(mr != null);    while (!mr.acquire(pages));    lock();    int page = freeList.alloc(pages);    if (page == -1) {      unlock();      return VM_Address.zero();    }    pagetotal += pages;    unlock();    VM_Address rtn = start.add(Conversions.pagesToBytes(page));    LazyMmapper.ensureMapped(rtn, Conversions.pagesToBlocks(pages));    return rtn;  }