Understanding the Linux Kernel, Second Edition

Chapter 7. Memory Management

We saw in Chapter 2 how Linux takes advantage of 80 × 86’s segmentation and paging circuits to translate logical addresses into physical ones. We also mentioned that some portion of RAM is permanently assigned to the kernel and used to store both the kernel code and the static kernel data structures.

The remaining part of the RAM is called dynamic memory . It is a valuable resource, needed not only by the processes but also by the kernel itself. In fact, the performance of the entire system depends on how efficiently dynamic memory is managed. Therefore, all current multitasking operating systems try to optimize the use of dynamic memory, assigning it only when it is needed and freeing it as soon as possible.

This chapter, which consists of three main sections, describes how the kernel allocates dynamic memory for its own use. Section 7.1 and Section 7.2 illustrate two different techniques for handling physically contiguous memory areas, while Section 7.3 illustrates a third technique that handles noncontiguous memory areas.

Page Frame Management

We saw in Section 2.4 how the Intel Pentium processor can use two different page frame sizes: 4 KB and 4 MB (or 2 MB if PAE is enabled—see Section 2.4.6). Linux adopts the smaller 4 KB page frame size as the standard memory allocation unit. This makes things simpler for two reasons:

• The Page Fault exceptions issued by the paging circuitry are easily interpreted. Either the page requested exists but the process ...