As we have learned in the previous chapter, ultimately there is just one engine for memory allocation within the kernel – the page (or buddy system) allocator. Layered on top is the slab allocator (or slab cache) machinery. In addition, there is another completely virtual address space within the kernel's address space from where virtual pages can be allocated at will – this is called the kernel vmalloc region.
Of course, ultimately, once a virtual page is actually used (by something in the kernel or in user space via a process or thread) - it's physical page frame that it's mapped to is really allocated via the page allocator (this is ultimately true of all user space memory frames as well, though in an indirect fashion; more on this later in the Demand paging and OOM section).
Within the kernel segment or VAS (we covered all this in some detail in Chapter 7, Memory Management...