Section 4.9. Page Tables

4.9. Page Tables

Program memory is comfortably managed with virtual addresses. The problem with this is that when an instruction is issued to the processor, it cannot do anything with a virtual address. The processor operates on physical addresses. The association between the virtual address and the corresponding physical address is kept by the kernel (with help from the hardware) in page tables.

Page tables keep track of memory in page frame units. They are stored in RAM throughout the kernel's lifespan. Linux has what is called a three-level paging scheme. Three-level paging is sufficient to ensure that 64-bit architectures have enough space to maintain mappings of all their virtual-to-physical associations. As the name implies, three-level paging has three types of paging tables: The top level directory is called the Page Global Directory (PGD) and is represented by a pgd_t datatype; the second page is called the Page Middle Directory (PMD) and is represented by a pmd_t datatype; the final page is called a Page Table (PTE) and is represented by a pte_t datatype. Figure 4.13 illustrates the page tables.

Figure 4.13. Page Tables in Linux

The PGD holds entries that refer to PMDs. The PMD holds entries that refer to PTEs, and the PTE holds entries that refer to specific pages. Each process has its own set of page tables. The mm_struct->pgd field points to the PGD for the process. The 32- or 64-bit virtual addresses are split up into variously sized (depending on the architecture) offset fields. Each field corresponds to an offset within the PGD, PMD, PTE, and the page itself.