Linux Kernel Architecture

Chapter 1: Introduction and Overview

Process Representation

RAM

Process Types

Page Frame

Figure 1-6: Virtual and physical addresses.

The arrows in Figure 1-6 indicate how the pages in thevirtual address spaces are distributed across the

physical pages. For example, virtual page 1 of process A is mapped to physical page 4, while virtual

page 1 of process B is mapped to the fifth physical page. This shows that virtual addresses change their

meaning from process to process.

Physical pages are often calledpage frames. In contrast, the termpageis reserved for pages in virtual

address space.

Mapping between virtual address spaces and physicalmemory also enables the otherwise strict sep-

aration between processes to be lifted. Our example includes a page frame explicitly shared by both

processes. Page 5 of A and page 1 of B both point to the physical page frame 5. This is possible because

entries in both virtual address spaces (albeit at different positions) point to the same page. Since the ker-

nel is responsible for mapping virtual address spaceto physical address space, it is able to decide which

memory areas are to be shared between processes and which are not.

The figure also shows that not all pages of the virtual address spaces are linked with a page frame. This

may be because either the pages are not used or because data have not been loaded into memory because

they are not yet needed. It may also be that the page has been swapped out onto hard disk and will be

swapped back in when needed.

Finally, notice that there are two equivalent terms to address the applications that run on behalf of the

user. One of them isuserland, and this is the nomenclature typically preferred by the BSD community for

all things that do not belong to the kernel. The alternative is to say that an application runs inuserspace.It

should be noted that the termuserlandwill always mean applications as such, whereas the termuserspace

can additionally not only denote applications, but also the portion of the virtual address space in which

they are executed, in contrast tokernel space.

1.3.4 Page Tables

Data structures known aspage tablesare used to map virtual address space to physical address space. The

easiest way of implementing the association between both would be to use an array containing an entry

for each page in virtual address space. This entry would point to the associated page frame. But there is

a problem. IA-32 architecture uses, for example, 4 KiB pages — given a virtual address space of 4 GiB,

this would produce an array with a million entries. On 64-bit architectures, the situation is much worse.

Because each process needs its own page tables, this approach is impractical because the entire RAM of

the system would be needed to hold the page tables.