Paging in Linux

As we explained earlier in Section 2.4.5, Linux adopted a three-level paging model so paging is feasible on 64-bit architectures. Figure 2-11 shows the model, which defines three types of paging tables.

• Page Global Directory

• Page Middle Directory

The Page Global Directory includes 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. The linear address is thus split into four parts. Figure 2-11 does not show the bit numbers because the size of each part depends on the computer architecture.

Figure 2-11. The Linux paging model

Figure 2-11. The Linux paging model

Linux's handling of processes relies heavily on paging. In fact, the automatic translation of linear addresses into physical ones makes the following design objectives feasible:

• Assign a different physical address space to each process, ensuring an efficient protection against addressing errors.

• Distinguish pages (groups of data) from page frames (physical addresses in main memory). This allows the same page to be stored in a page frame, then saved to disk and later reloaded in a different page frame. This is the basic ingredient of the virtual memory mechanism (see Chapter 16).

As we shall see in Chapter 8, each process has its own Page Global Directory and its own set of Page Tables. When a process switch occurs (see Section 3.3), Linux saves the cr3 control register in the descriptor of the process previously in execution and then loads cr3 with the value stored in the descriptor of the process to be executed next. Thus, when the new process resumes its execution on the CPU, the paging unit refers to the correct set of Page Tables.

What happens when this three-level paging model is applied to the Pentium, which uses only two types of Page Tables? Linux essentially eliminates the Page Middle Directory field by saying that it contains zero bits. However, the position of the Page Middle Directory in the sequence of pointers is kept so that the same code can work on 32-bit and 64-bit architectures. The kernel keeps a position for the Page Middle Directory by setting the number of entries in it to 1 and mapping this single entry into the proper entry of the Page Global Directory.

However, when Linux uses the Physical Address Extension (PAE) mechanism of the Pentium Pro and later processors, the Linux's Page Global Directory corresponds to the 80 x 86's Page Directory Pointer Table, the Page Middle Directory to the 80 x 86's Page Directory, and the Linux's Page Table to the 80 x 86's Page Table.

Mapping logical to linear addresses now becomes a mechanical task, although it is still somewhat complex. The next few sections of this chapter are a rather tedious list of functions and macros that retrieve information the kernel needs to find addresses and manage the tables; most of the functions are one or two lines long. You may want to just skim these sections now, but it is useful to know the role of these functions and macros because you'll see them often in discussions throughout this book.

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