Specifications

This is the Title of the Book, eMatter Edition

416

Chapter 15: Memory Mapping and DMA

needed for the kernel code itself. As a result, x86-based Linux systems could work

with a maximum of a little under 1 GB of physical memory.

In response to commercial pressure to support more memory while not breaking 32-

bit application and the system’s compatibility, the processor manufacturers have

added “address extension” features to their products. The result is that, in many

cases, even 32-bit processors can address more than 4 GB of physical memory. The

limitation on how much memory can be directly mapped with logical addresses

remains, however. Only the lowest portion of memory (up to 1 or 2 GB, depending

on the hardware and the kernel configuration) has logical addresses;

the rest (high

memory) does not. Before accessing a specific high-memory page, the kernel must set

up an explicit virtual mapping to make that page available in the kernel’s address

space. Thus, many kernel data structures must be placed in low memory; high mem-

ory tends to be reserved for user-space process pages.

The term “high memory” can be confusing to some, especially since it has other

meanings in the PC world. So, to make things clear, we’ll define the terms here:

Low memory

Memory for which logical addresses exist in kernel space. On almost every sys-

tem you will likely encounter, all memory is low memory.

High memory

Memory for which logical addresses do not exist, because it is beyond the

address range set aside for kernel virtual addresses.

On i386 systems, the boundary between low and high memory is usually set at just

under 1 GB, although that boundary can be changed at kernel configuration time.

This boundary is not related in any way to the old 640 KB limit found on the origi-

nal PC, and its placement is not dictated by the hardware. It is, instead, a limit set by

the kernel itself as it splits the 32-bit address space between kernel and user space.

We will point out limitations on the use of high memory as we come to them in this

chapter.

The Memory Map and Struct Page

Historically, the kernel has used logical addresses to refer to pages of physical mem-

ory. The addition of high-memory support, however, has exposed an obvious prob-

lem with that approach—logical addresses are not available for high memory.

Therefore, kernel functions that deal with memory are increasingly using pointers to

struct page (defined in <linux/mm.h>) instead. This data structure is used to keep

track of just about everything the kernel needs to know about physical memory;

* The 2.6 kernel (with an added patch) can support a “4G/4G” mode on x86 hardware, which enables larger

kernel and user virtual address spaces at a mild performance cost.

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