Advances in PCI-Express and optical interconnects are making rack-scale computers possible, but these computers will undoubtedly exhibit Non-Uniform Memory Access (NUMA) latencies. Ideally, a hypervisor for rack-scale computers should be able to dynamically reconfigure a virtual machine’s processing and memory resources, i.e., its NUMA topology, to satisfy each application’s evolving demands. Unfortunately, current hypervisors lack support for such dynamic reconfiguration. To that end, this paper introduces Virtflex, a multilayered system for enabling unmodified OpenMP applications to adapt automatically to NUMA topology changes. Virtflex provides a novel NUMA page placement reset mechanism within the guest OS and a novel NUMA-aware superpage ballooning mechanism that spans the guest OS-hypervisor boundary. The evaluation shows that Virtflex enables applications to adapt efficiently to NUMA topology changes. For example, adding resources incurs an average runtime overhead of only 7.27%.