1. A data structure used by a memory management unit of a computer system to manage mappings of virtual addresses to physical memory pages, the data structure comprising: a set of tables that are hierarchically arranged, the set of tables including a root table and a plurality of bottom-level tables, wherein each of a plurality of entries of the bottom-level tables references a physical memory page in common with at least one other entry of the bottom-level tables, wherein each entry of the bottom-level tables includes a status bit value that indicates whether or not data stored in the physical memory page referenced by said entry is dirty or not, and said status bit values of at least two entries of the bottom-level tables that reference the same physical memory page are different.