Linux Kernel Architecture

Chapter3:MemoryManagement

1

Page tables

Page tables

Physical memory

Task switch

3

Kernel

User

Kernel

User

Figure 3-14: Connection between virtual and physical

address space on IA-32 processors.

Division of Address Space

Division of address space in a ratio of 3 : 1 is only an approximate reflection of the situation in the kernel

as the kernel address space itself is split into various sections. Figure 3-15 graphically illustrates the

situation.

high_memory VMALLOC_START VMALLOC_END

_ _PAGE_OFFSET

0xC000000

8 MiB

PKMAP_BASE

FIXADDR_START 4 GiB

Mapping of all

physical page

frames

VMALLOC

MappingsPersistentFixmaps

Figure 3-15: Division of the kernel address space on IA-32 systems.

The figure shows the structure of the pagetable entries used to manage the fourth

gigabyte of virtual address space. It indicates thepurposeof each area ofvirtual

address space, and this has nothing todo with the assignment of physical RAM.

The first section of the address space is used to map all physical pages of the system into the virtual

address space of the kernel. Because this address space begins at an offset of0xC0000000 —thefre-

quently mentioned 3 GiB — each virtual addressxcorresponds to the physical addressx− 0 xC0000000,

and is therefore a simple linear shift.

As the figure shows, thedirect mappingarea extends from0xC0000000to thehigh_memoryaddress whose

exact value I discuss shortly. As intimated in Chapter 1, there is a problem with this scheme. Because the

virtual address space of the kernel comprises only 1 GiB, a maximum of 1 GiB of RAM memory can be

mapped. The fact that the maximum memory configuration on IA-32 systems (without PAE) can be up

to 4 GiB raises the question of what to do with the remaining memory.

Here’s the bad news. The kernel cannot map the whole of physical memory at once if it is larger

than 896 MiB.^13 This value is even less than the previously stated maximum limit of 1 GiB because

(^13) It would also be possible to get rid of the split completely by introducingtwo4 GiB address spaces, one for the kernel and one for
each userspace program. However, context switches between kernel and user mode are more costly in this case.