1. A method for oversubscribing a host memory of a host running a virtual machine monitor (VMM), comprising: examining a virtual machine (VM) memory for the VM for metadata associated with the VM memory, the metadata including information from a physical frame number (PFN) database extracted from a kernel of the guest OS, the PFN database maintaining state information for each PFN the guest OS the VM knows about; collecting the metadata for the VM memory; and managing the VM memory using the metadata for oversubscribing a host memory.