Understanding the Linux Kernel

Chapter 7. Process Address Space

As seen in the previous chapter, a kernel function gets dynamic memory in a fairly straightforward manner by invoking one of a variety of functions: __get_free_pages( ) to get pages from the buddy system algorithm, kmem_cache_alloc( ) or kmalloc( ) to use the slab allocator for specialized or general-purpose objects, and vmalloc( ) to get a noncontiguous memory area. If the request can be satisfied, each of these functions returns a linear address identifying the beginning of the allocated dynamic memory area.

These simple approaches work for two reasons:

• The kernel is the highest priority component of the operating system: if some kernel function makes a request for dynamic memory, it must have some valid reason to issue that request, and there is no point in trying to defer it.

• The kernel trusts itself: all kernel functions are assumed error-free, so it does not need to insert any protection against programming errors.

When allocating memory to User Mode processes, the situation is entirely different:

• Process requests for dynamic memory are considered nonurgent. When a process's executable file is loaded, for instance, it is unlikely that the process will address all the pages of code in the near future. Similarly, when a process invokes malloc( ) to get additional dynamic memory, it doesn't mean the process will soon access all the additional memory obtained. So as a general rule, the kernel tries to defer allocating dynamic memory to User ...