Linux内存管理系统详解：权威入门指南

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本文档深入探讨了Linux内存管理系统，作为一本桌面参考书籍，它基于Linux内核版本2.4.19，由Abhishek Nayani、Mel Gorman和Rodrigo S. de Castro合作编写，发布日期为2002年5月25日。作者在书中详细介绍了Linux内存管理的关键组成部分，包括初始化过程、内存检测方法（如E820和E801H）、GDT（全局描述符表）的设置与激活，以及页表管理和最终内存检测策略。首先，内存检测是系统启动初期的重要步骤，文档列举了三种方法来确定系统的内存布局：E820方法用于检测BIOS提供的物理内存信息；E801H方法可能是对早期BIOS标准的支持；而88H方法可能与特定硬件配置有关。在初始化过程中，临时全局描述符表（Provisional GDT）被用来设置基本的内存保护和寻址结构。 Paging机制的激活是关键环节，其中PAGEOFFSET字段的含义至关重要，它确定了页表的偏移量，对于正确地组织和访问内存空间至关重要。在此阶段，还会创建临时的内核页表，随着系统运行，这些页表会被替换为最终的页表结构，从而实现虚拟内存功能。 “setuparch()”、“setupmemoryregion()”等函数在内存检测和初始化过程中扮演重要角色，它们处理内存区域的识别和配置，确保内存的有效管理。此外，文档还讨论了“sanitizee820map()”、“copye820map()”和“addmemoryregion()”等函数，这些函数用于处理和整合来自BIOS的信息，进一步完善内存映射。最后，文章提到的“MemoryDetectionRevisited”部分着重于内存检测的重新审视，可能涉及对不同内存配置处理策略的更新或优化，以适应不断变化的硬件环境。整个章节强调了Linux内存管理的灵活性和动态调整能力，这对于理解现代操作系统如何高效利用和保护系统资源至关重要。总结起来，本资源提供了深入剖析Linux内存管理系统的宝贵信息，适合对内核源代码、内存管理有深入了解的开发者和系统管理员参考，无论是内存检测技术、页表管理还是内存区域配置，都能从中获得有价值的洞见。

4 CHAPTER 1. INITIALIZATION

AX = number of contiguous KB starting

at absolute address 100000h

CF set on error

AH = status

80h invalid command (PC,PCjr)

86h unsupported function (XT,PS30)

The size calculated is stored in the address location 0x2h.

1.2 Provisional GDT

Before entering protected mode, the global descriptor table has to be setup.

A provisional or temporary gdt is created with two entries, code and data

segment, each covering the whole 4GB address space. The code

that loads

the gdt is:

/** /arch/i386/boot/setup.S **/

xorl %eax, %eax # Compute gdt_base

movw %ds, %ax # (Convert %ds:gdt to a linear ptr)

shll $4, %eax

addl $gdt, %eax

movl %eax, (gdt_48+2)

lgdt gdt_48 # load gdt with whatever is

# appropriate

where the variable gdt contains the table, gdt 48 contains the limit and

the address of gdt. The code above gets the address of gdt and ﬁlls it in the

address part of the gdt 48 variable.

1.3 Activating Paging

1.3.1 Signiﬁcance of PAGE OFFSET

The value of PAGE OFFSET is 0xc0000000 which is 3GB. The linear address

space of a process is divided into two parts:

In ﬁle arch/i386/kernel/head.S

1.3. ACTIVATING PAGING 5

• Linear addresses from 0x00000000 to PAGE OFFSET-1 can be ad-

dressed when the process is either in user or kernel mode.

• Linear addresses from PAGE OFFSET to 0xﬀﬀﬀﬀ can be addressed

only when the process is in kernel mode. This address space is common

to all the processes.

The address space after PAGE OFFSET is reserved for the kernel and

this is where the complete physical memory is mapped (eg. if a system

has 64mb of RAM, it is mapped from PAGE OFFSET to PAGE OFFSET

+ 64mb). This address space is also used to map non-continuous physical

memory into continuous virtual memory.

1.3.2 Provisional Kernel Page Tables

The purpose of this page directory is to map virtual address spaces 0–8mb

and PAGE OFFSET–(PAGE OFFSET + 8mb) to the physical address space

of 0–8mb. This mapping is done so that the address space out of which the

code is executing, remains valid. Joseph A Knapka has explained this much

better, from which I quote:

• All pointers in the compiled kernel refer to addresses > PAGE -

OFFSET. That is, the kernel is linked under the assumption that its

base address will be start text (I think; I don’t have the code on hand

at the moment), which is deﬁned to be PAGE OFFSET+(some small

constant, call it C).

• All the kernel bootstrap code is linked assuming that its base address is

0+C.

head.S is part of the bootstrap code. It’s running in protected mode with

paging turned oﬀ, so all addresses are physical. In particular, the instruction

pointer is fetching instructions based on physical address. The instruction

that turns on paging (movl %eax, %cr0) is located, say, at some physical

address A.

As soon as we set the paging bit in cr0, paging is enabled, and starting at

the very next instruction, all addressing, including instruction fetches, pass

through the address translation mechanism (page tables). IOW, all address

are henceforth virtual. That means that

1. We must have valid page tables, and

2. Those tables must properly map the instruction pointer to the next in-

struction to be executed.

6 CHAPTER 1. INITIALIZATION

That next instruction is physically located at address A+4 (the address

immediately after the ”movl %eax, %cr0” instruction), but from the point of

view of all the kernel code – which has been linked at PAGE OFFSET – that

instruction is located at virtual address PAGE OFFSET+(A+4). Turning

on paging, however, does not magically change the value of EIP. The CPU

fetches the next instruction from ***virtual*** address A+4; that instruction

is the beginning of a short sequence that eﬀectively relocates the instruction

pointer to point to the code at PAGE OFFSET+A+(something).

But since the CPU is, for those few instructions, fetching instructions

based on physical addresses ***but having those instructions pass through

address translation***, we must ensure that both the physical addresses and

the virtual addresses are :

1. Valid virtual addresses,and

2. Point to the same code.

That means that at the very least, the initial page tables must map virtual

address PAGE OFFSET+(A+4) to physical address (A+4), and must map

virtual address A+4 to physical address A+4. This dual mapping for the ﬁrst

8MB of physical RAM is exactly what the initial page tables accomplish. The

8MB initially mapped is more or less arbitrary. It’s certain that no bootable

kernel will be greater than 8MB in size. The identity mapping is discarded

when the MM system gets initialized.

The variable swapper pg dir contains the page directory for the kernel, which

is statically initialized at compile time. Using “.org” directives of the assem-

bler, swapper pg dir is placed at address 0x00101000

, similarly the ﬁrst page

table entry pg0 is placed at 0x00102000 and the second page table entry pg1

at 0x00103000. The page table entry pg1 is followed by empty zero page

0x00103000, whose only purpose is to act as a marker to denote the end, in

a loop used to initialize the page tables. The swapper pg dir is as follows:

/** /arch/i386/kernel/head.S **/

.org 0x1000

ENTRY(swapper_pg_dir)

.long 0x00102007

.long 0x00103007

The kernel starts at 0x00100000 == 1MB, so .org 0x1000 is taken w.r.t the start of

the kernel

It is also used to store the boot parameters and the command line of the kernel.

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thunderpower

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