Linux Device Drivers, 3rd Edition

Chapter 8. Allocating Memory

Thus far, we have used kmalloc and kfree for the allocation and freeing of memory. The Linux kernel offers a richer set of memory allocation primitives, however. In this chapter, we look at other ways of using memory in device drivers and how to optimize your system’s memory resources. We do not get into how the different architectures actually administer memory. Modules are not involved in issues of segmentation, paging, and so on, since the kernel offers a unified memory management interface to the drivers. In addition, we won’t describe the internal details of memory management in this chapter, but defer it to Chapter 15.

The Real Story of kmalloc

The kmalloc allocation engine is a powerful tool and easily learned because of its similarity to malloc. The function is fast (unless it blocks) and doesn’t clear the memory it obtains; the allocated region still holds its previous content.^[1] The allocated region is also contiguous in physical memory. In the next few sections, we talk in detail about kmalloc, so you can compare it with the memory allocation techniques that we discuss later.

The Flags Argument

Remember that the prototype for kmalloc is:

#include <linux/slab.h>

void *kmalloc(size_t size, int flags);

The first argument to kmalloc is the size of the block to be allocated. The second argument, the allocation flags, is much more interesting, because it controls the behavior of kmalloc in a number of ways.

The most commonly used flag, GFP_KERNEL ...