Linux Kernel Architecture

Chapter3:MemoryManagement

should be reported by a system error message rather than allowing the data of other kernel parts to be

overwritten unnoticed. Because this separation is made in virtual address space, no valuable real memory

pages are wasted.

Reserving Memory withvmalloc

vmallocis the interface function used by the kernel code to request memory that is not necessarily con-

tiguous in physical memory but is always linear in virtual memory.

<vmalloc.h>

void *vmalloc(unsigned long size);

Just one parameter is required to specify the size of the required memory area — in contrast to the func-

tions discussed earlier, the size unit is not pages but bytes, as is common in userspace programming.

The best-known example ofvmallocuse is in the module implementation of the kernel. Because modules

can be loaded at any time, there is no guarantee — particularly if the system has been up and running for

a long time — that sufficient contiguous memory will be available for the sometimes voluminous module

data. This problem can be circumvented by usingvmallocif sufficient memory can be pieced together

from smaller chunks.

vmallocis also invoked at about 400 other places in the kernel, particularly in device and sound drivers.

Because the memory pages used forvmallocmust in any case be actively mapped in kernel address

space, it is obviously preferable to use pages fromZONE_HIGHMEMfor this purpose. This allows the

kernel to conserve the more valuable lower zones without incurring any added disadvantages. For

this reason,vmalloc(along with the mapping functions discussed in Section 3.5.8) is one of the few

occasions when the kernel is able to use highmem pages for its own purposes (and not for userspace

applications).

Data Structures

When it manages thevmallocarea in virtual memory, the kernel must keep track of which sections

are in use and which are free. To this end, it defines a data structure to hold all used sections in a

linked list.

The kernel uses an important data structure calledvm_area_structto manage the

virtual address space contents of a userspace process. Despite the similarity of name

and purpose, these two structures must not be confused.

<vmalloc.h>

struct vm_struct {

struct vm_struct *next;

void *addr;

unsigned long size;

unsigned long flags;

struct page **pages;

unsigned int nr_pages;

unsigned long phys_addr;

};