Linux Kernel Architecture

Chapter3:MemoryManagement

The functions often appear in groups of three to set, delete, and query a specific attribute, for instance,

write permission for a page. The kernel assumes that access to page data can be regulated in three dif-

ferent ways — by means of write, read, and execution permission. (Execution permission indicates that

page binary data may be executed as machine code in the same way as programs are executed.) How-

ever, this assumption is a little too optimistic for some CPUs. IA-32 processors support only two control

modes to allow reading and writing. In this case, the architecture-dependent code tries to emulate the

desired semantics as best it can.

3.3.2 Creating and Manipulating Entries

Table 3-4 lists all functions for creating new page table entries.

Table 3-4: Functions for Creating New Page Table Entries

Function Description

mk_pte

Createsapteentry;apageinstance and the desired page access permissions must

be passed as parameters.

pte_page

Yields the address of thepageinstance belonging to the page described by the

page table entry.

pgd_alloc

pud_alloc

pmd_alloc

pte_alloc

Reserve and initialize memory to hold a complete page table (not just a single

entry).

pgd_free

pud_free

pmd_free

pte_free

Free the memory occupied by the page table.

set_pgd

set_pud

set_pmd

set_pte

Set the value of an entry in a page table.

The functions in the table must be implemented by allarchitectures to enable memory management code

to create and destroy page tables.

3.4 Initialization of Memory Management

In the context of memory management,initializationcan have multiple meanings. On many CPUs, it is

necessary to explicitly set the memory model suitable for the Linux kernel, for example, by switching

toprotected modeon IA-32 systems, before it is possible to detect the available memory and register it

with the kernel. In the course of initialization, it is also necessary to set up the data structures of memory

management, and much more. Because the kernel needs memory before memory management is fully