Linux adopts a common paging model that fits both 32-bit and 64-bit architectures. As explained in the earlier section "Paging for 64-bit Architectures,” two paging levels are sufficient for 32-bit architectures, while 64-bit architectures require a higher number of paging levels. Up to version 2.6.10, the Linux paging model consisted of three paging levels. Starting with version 2.6.11, a four-level paging model has been adopted.^[*] The four types of page tables illustrated in Figure 2-12 are called:

The Page Global Directory includes the addresses of several Page Upper Directories, which in turn include the addresses of several Page Middle Directories, which in turn include the addresses of several Page Tables. Each Page Table entry points to a page frame. Thus the linear address can be split into up to five parts. Figure 2-12 does not show the bit numbers, because the size of each part depends on the computer architecture.

For 32-bit architectures with no Physical Address Extension, two paging levels are sufficient. Linux essentially eliminates the Page Upper Directory and the Page Middle Directory fields by saying that they contain zero bits. However, the positions of the Page Upper Directory and the Page Middle Directory in the sequence of pointers are kept so that the same code can work on 32-bit and 64-bit architectures. The kernel keeps a position for the Page Upper Directory and the Page ...