Understanding the Linux Kernel

7.1. The Process's Address Space

The address space of a process consists of all linear addresses that the process is allowed to use. Each process sees a different set of linear addresses; the address used by one process bears no relation to the address used by another. As we shall see later, the kernel may dynamically modify a process address space by adding or removing intervals of linear addresses.

The kernel represents intervals of linear addresses by means of resources called memory regions, which are characterized by an initial linear address, a length, and some access rights. For reasons of efficiency, both the initial address and the length of a memory region must be multiples of 4096, so that the data identified by each memory region entirely fills up the page frames allocated to it. Let us briefly mention typical situations in which a process gets new memory regions:

• When the user types a command at the console, the shell process creates a new process to execute the command. As a result, a fresh address space, thus a set of memory regions, is assigned to the new process (see Section 7.5 later in this chapter and Chapter 19).

• A running process may decide to load an entirely different program. In this case, the process ID remains unchanged but the memory regions used before loading the program are released, and a new set of memory regions is assigned to the process (see Section 19.4 in Chapter 19).

• A running process may perform a "memory mapping" on a file (or on a portion ...