Linux Device Drivers, Second Edition

The mm Directory

The last major directory of kernel source files is devoted to memory management. The files in this directory implement all the data structures that are used throughout the system to manage memory-related issues. While memory management is founded on registers and features specific to a given CPU, we’ve already seen in Chapter 13 how most of the code has been made platform independent. Interested users can check how asm/arch- arch /mm implements the lowest level for a specific computer platform.

The kmalloc/kfree memory allocation engine is defined in slab.c. This file is a completely new implementation that replaces what used to live in kmalloc.c. The latter file doesn’t exist anymore after version 2.0.

While most programmers are familiar with how an operating system manages memory in blocks and pages, Linux (taking an idea from Sun Microsystem’s Solaris) uses an additional, more flexible concept called a slab. Each slab is a cache that contains multiple memory objects of the same size. Some slabs are specialized and contain structs of a certain type used by a certain part of the kernel; others are more general and contain memory regions of 32 bytes, 64 bytes, and so on. The advantage of using slabs is that structs or other regions of memory can be cached and reused with very little overhead; the more ponderous technique of allocating and freeing pages is invoked less often.

The other important allocation tool, vmalloc, and the function that lies behind ...