深入理解Linux内核：Kindle适配版

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"理解Linux内核——适用于Kindle Oasis的版本" 《理解Linux内核》是针对Linux操作系统的一本深度技术书籍，特别优化了Kindle Oasis阅读体验，可以直接在PDF上阅读，方便所有使用Kindle设备的读者。这本书深入探讨了Linux内核的工作原理，对于想要深入了解Linux系统运行机制的人来说是一份宝贵的资源。 Linux内核是操作系统的核心，它负责处理CPU与外部世界的交互，包括硬件设备、网络通信等。内核决定了哪些程序可以使用处理器，并决定它们的执行顺序。它高效地管理有限的内存，使得数百个进程能够在系统中有效地共享资源，避免了资源的浪费和冲突。此外，内核还精心组织数据传输，确保CPU不会因等待较慢的硬盘数据而过多地浪费时间。《理解Linux内核》第三版带你深入探索内核中最重要的数据结构、算法以及编程技巧。作者不仅揭示了表面功能背后的深层运作机制，还为那些希望了解机器内部真正工作方式的人提供了有价值的见解。书中特别讨论了针对Intel平台的一些关键特性，并逐行解析了相关的代码段，使读者能够更直观地理解内核的实现细节。本书覆盖了广泛的内核功能，包括但不限于进程管理、内存管理、文件系统、网络协议栈、设备驱动程序等。通过阅读，你可以了解到： 1. 进程管理：如何创建、调度和终止进程，以及如何在进程间进行通信。 2. 内存管理：虚拟内存的概念，页面分配和回收策略，以及如何实现内存保护。 3. 调度算法：Linux内核采用的不同调度策略，如CFS（Completely Fair Scheduler）的公平调度。 4. 文件系统：VFS（Virtual File System）如何抽象出统一的文件访问接口，以及具体文件系统的实现，如EXT4。 5. 网络协议栈：从网络接口层到应用层的数据传输过程，TCP/IP协议的实现细节。 6. 设备驱动：如何编写和管理设备驱动程序，以使硬件与内核通信。《理解Linux内核》是一本全面、深入的技术指南，无论你是Linux开发者、系统管理员还是对操作系统有浓厚兴趣的学习者，都能从中受益匪浅，进一步提升对Linux内核及其运行机制的理解。

xiv

Preface

Overview of the Book

To make life easier, Chapter 1, Introduction, presents a general picture of what is

inside a Unix kernel and how Linux competes against other well-known Unix systems.

The heart of any Unix kernel is memory management. Chapter 2, Memory Addressing,

explains how 80×86 processors include special circuits to address data in memory and

how Linux exploits them.

Processes are a fundamental abstraction offered by Linux and are introduced in

Chapter 3, Processes. Here we also explain how each process runs either in an unprivi-

leged User Mode or in a privileged Kernel Mode. Transitions between User Mode and

Kernel Mode happen only through well-established hardware mechanisms called inter-

rupts and exceptions. These are introduced in Chapter 4, Interrupts and Exceptions.

In many occasions, the kernel has to deal with bursts of interrupt signals coming from

different devices and processors. Synchronization mechanisms are needed so that all

these requests can be serviced in an interleaved way by the kernel: they are discussed in

Chapter 5, Kernel Synchronization, for both uniprocessor and multiprocessor systems.

One type of interrupt is crucial for allowing Linux to take care of elapsed time; fur-

ther details can be found in Chapter 6, Timing Measurements.

Chapter 7, Process Scheduling, explains how Linux executes, in turn, every active

process in the system so that all of them can progress toward their completions.

Next we focus again on memory. Chapter 8, Memory Management, describes the

sophisticated techniques required to handle the most precious resource in the sys-

tem (besides the processors, of course): available memory. This resource must be

granted both to the Linux kernel and to the user applications. Chapter 9, Process

Address Space, shows how the kernel copes with the requests for memory issued by

greedy application programs.

Chapter 10, System Calls, explains how a process running in User Mode makes

requests to the kernel, while Chapter 11, Signals, describes how a process may send

synchronization signals to other processes. Now we are ready to move on to another

essential topic, how Linux implements the filesystem. A series of chapters cover this

topic. Chapter 12, The Virtual Filesystem, introduces a general layer that supports

many different filesystems. Some Linux files are special because they provide trap-

doors to reach hardware devices; Chapter 13, I/O Architecture and Device Drivers,

and Chapter 14, Block Device Drivers, offer insights on these special files and on the

corresponding hardware device drivers.

Another issue to consider is disk access time; Chapter 15, The Page Cache, shows

how a clever use of RAM reduces disk accesses, therefore improving system perfor-

mance significantly. Building on the material covered in these last chapters, we can

now explain in Chapter 16, Accessing Files, how user applications access normal

files. Chapter 17, Page Frame Reclaiming, completes our discussion of Linux mem-

ory management and explains the techniques used by Linux to ensure that enough

xvi

Preface

To comment or ask technical questions about this book, send email to:

bookquestions@oreilly.com

For more information about our books, conferences, Resource Centers, and the

O’Reilly Network, see our web site at:

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Acknowledgments

This book would not have been written without the precious help of the many stu-

dents of the University of Rome school of engineering “Tor Vergata” who took our

course and tried to decipher lecture notes about the Linux kernel. Their strenuous

efforts to grasp the meaning of the source code led us to improve our presentation

and correct many mistakes.

Andy Oram, our wonderful editor at O’Reilly Media, deserves a lot of credit. He was

the first at O’Reilly to believe in this project, and he spent a lot of time and energy

deciphering our preliminary drafts. He also suggested many ways to make the book

more readable, and he wrote several excellent introductory paragraphs.

We had some prestigious reviewers who read our text quite carefully. The first edi-

tion was checked by (in alphabetical order by first name) Alan Cox, Michael Kerrisk,

Paul Kinzelman, Raph Levien, and Rik van Riel.

The second edition was checked by Erez Zadok, Jerry Cooperstein, John Goerzen,

Michael Kerrisk, Paul Kinzelman, Rik van Riel, and Walt Smith.

This edition has been reviewed by Charles P. Wright, Clemens Buchacher, Erez

Zadok, Raphael Finkel, Rik van Riel, and Robert P. J. Day. Their comments, together

with those of many readers from all over the world, helped us to remove several

errors and inaccuracies and have made this book stronger.

—Daniel P. Bovet

Marco Cesati

July 2005

CHAPTER 1

Introduction

Linux

is a member of the large family of Unix-like operating systems. A relative new-

comer experiencing sudden spectacular popularity starting in the late 1990s, Linux

joins such well-known commercial Unix operating systems as System V Release 4

(SVR4), developed by AT&T (now owned by the SCO Group); the 4.4 BSD release

from the University of California at Berkeley (4.4BSD); Digital UNIX from Digital

Equipment Corporation (now Hewlett-Packard); AIX from IBM; HP-UX from

Hewlett-Packard; Solaris from Sun Microsystems; and Mac OS X from Apple Com-

puter, Inc. Beside Linux, a few other opensource Unix-like kernels exist, such as

FreeBSD, NetBSD, and OpenBSD.

Linux was initially developed by Linus Torvalds in 1991 as an operating system for

IBM-compatible personal computers based on the Intel 80386 microprocessor. Linus

remains deeply involved with improving Linux, keeping it up-to-date with various

hardware developments and coordinating the activity of hundreds of Linux develop-

ers around the world. Over the years, developers have worked to make Linux avail-

able on other architectures, including Hewlett-Packard’s Alpha, Intel’s Itanium,

AMD’s AMD64, PowerPC, and IBM’s zSeries.

One of the more appealing benefits to Linux is that it isn’t a commercial operating

system: its source code under the GNU General Public License (GPL)

†

is open and

available to anyone to study (as we will in this book); if you download the code (the

official site is http://www.kernel.org) or check the sources on a Linux CD, you will be

able to explore, from top to bottom, one of the most successful modern operating

systems. This book, in fact, assumes you have the source code on hand and can

apply what we say to your own explorations.

* LINUX® is a registered trademark of Linus Torvalds.

† The GNU project is coordinated by the Free Software Foundation, Inc. (http://www.gnu.org); its aim is to

implement a whole operating system freely usable by everyone. The availability of a GNU C compiler has

been essential for the success of the Linux project.

Chapter

1: Introduction

Technically speaking, Linux is a true Unix kernel, although it is not a full Unix operat-

ing system because it does not include all the Unix applications, such as filesystem

utilities, windowing systems and graphical desktops, system administrator com-

mands, text editors, compilers, and so on. However, because most of these programs

are freely available under the GPL, they can be installed in every Linux-based system.

Because the Linux kernel requires so much additional software to provide a useful

environment, many Linux users prefer to rely on commercial distributions, available on

CD-ROM, to get the code included in a standard Unix system. Alternatively, the code

may be obtained from several different sites, for instance http://www.kernel.org. Sev-

eral distributions put the Linux source code in the /usr/src/linux directory. In the rest of

this book, all file pathnames will refer implicitly to the Linux source code directory.

Linux Versus Other Unix-Like Kernels

The various Unix-like systems on the market, some of which have a long history and

show signs of archaic practices, differ in many important respects. All commercial

variants were derived from either SVR4 or 4.4BSD, and all tend to agree on some

common standards like IEEE’s Portable Operating Systems based on Unix (POSIX)

and X/Open’s Common Applications Environment (CAE).

The current standards specify only an application programming interface (API)—

that is, a well-defined environment in which user programs should run. Therefore,

the standards do not impose any restriction on internal design choices of a compli-

ant kernel.

To define a common user interface, Unix-like kernels often share fundamental design

ideas and features. In this respect, Linux is comparable with the other Unix-like

operating systems. Reading this book and studying the Linux kernel, therefore, may

help you understand the other Unix variants, too.

The 2.6 version of the Linux kernel aims to be compliant with the IEEE POSIX stan-

dard. This, of course, means that most existing Unix programs can be compiled and

executed on a Linux system with very little effort or even without the need for

patches to the source code. Moreover, Linux includes all the features of a modern

Unix operating system, such as virtual memory, a virtual filesystem, lightweight pro-

cesses, Unix signals, SVR4 interprocess communications, support for Symmetric

Multiprocessor (SMP) systems, and so on.

When Linus Torvalds wrote the first kernel, he referred to some classical books on

Unix internals, like Maurice Bach’s The Design of the Unix Operating System (Pren-

tice Hall, 1986). Actually, Linux still has some bias toward the Unix baseline

* As a matter of fact, several non-Unix operating systems, such as Windows NT and its descendents, are

POSIX-compliant.

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深入理解Linux内核：Kindle适配版

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