深入解析Linux虚拟内存管理

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"深入理解Linux虚拟内存管理" 在操作系统中，虚拟内存管理是核心功能之一，尤其是在Linux系统中。Linux内核的虚拟内存管理（VM）模块负责为进程分配和管理内存，确保高效且可靠地使用系统资源。《Understanding the Linux Virtual Memory Manager》一书详细阐述了这一主题，从概念到实现，对这一复杂但至关重要的系统进行了深入探讨。虚拟内存管理的主要任务包括： 1. **地址空间映射**：Linux使用虚拟地址空间，每个进程都有自己独立的地址空间。虚拟内存管理负责将这些虚拟地址映射到物理内存或磁盘上的交换空间。 2. **页面分配与释放**：当进程请求内存时，VM需要动态地分配页面。同时，它还需要回收不再使用的页面，以避免内存浪费。 3. **页面缓存**：为了提高I/O性能，Linux会缓存文件内容在内存中。VM管理这些缓存页面，平衡内存使用和I/O效率。 4. **页表管理**：每个进程的虚拟地址到物理地址的映射是通过页表实现的。VM需要维护这些页表，确保正确更新并进行内存访问权限控制。 5. **页面替换算法**：当物理内存不足时，VM需要选择一些页面写回磁盘，并释放其对应的物理内存。常见的页面替换算法有LRU（最近最少使用）、LFU（最不经常使用）等。 6. **内存压缩**：在内存极度紧张的情况下，Linux内核可能会采用内存压缩技术，将部分页面压缩后存储在内存中，以节省空间。 7. **交换机制**：当物理内存不足时，Linux会将不活跃的进程或数据交换到磁盘上的交换分区，以便为其他任务腾出内存。 8. **内存保护**：虚拟内存管理还提供了内存保护机制，防止一个进程访问其他进程的内存，从而保证系统的安全性和稳定性。 9. **内存锁定**：对于需要高速访问且不能被交换出去的数据，可以使用内存锁定功能，使其始终保留在物理内存中。 10. **内存泄漏检测**：虽然这不是VM的直接职责，但理解内存管理有助于开发者避免和诊断内存泄漏问题。书中详细讨论了这些机制的实现细节，如页高速缓存（Page Cache）、slab分配器、伙伴系统等，以及如何通过内核参数调整内存管理策略。此外，还涵盖了与虚拟内存相关的性能分析工具和调优技术，帮助系统管理员和开发者优化系统的内存使用。《Understanding the Linux Virtual Memory Manager》为读者提供了一个全面了解Linux内核虚拟内存管理的平台，无论你是系统管理员、开发人员还是研究人员，都能从中受益匪浅。

xvi Preface

• A live version of CodeViz, the tool used to generate call graphs for the book,

is available. If you feel that the book’s graphs are lacking some detail you

want,generate them yourself.

• The VMRegress, CodeViz and PatchSet packages, which are discussed

in Chapter 1, are available in /cdrom/software. gcc-3.0.4 is also provided

because it is required for building CodeViz.

Mount the CD on /cdrom as follows:

root@joshua:/$ mount /dev/cdrom /cdrom -o exec

The Web server is Apache 1.3.27 (http://www.apache.org/)andhas been built

and conﬁgured to run with its root as /cdrom/.Ifyour distribution normally uses

another directory, you will need to use this one instead. To start it, run the script

/cdrom/start

server.Ifnoerrors occur, the output should look like:

mel@joshua:~$ /cdrom/start_server

Starting CodeViz Server: done

Starting Apache Server: done

The URL to access is http://localhost:10080/

When the server starts successfully, point your browser to http://localhost:10080

to avail of the CD’s Web services. To shut down the server, run the script

/cdrom/stop

server,andtheCDmay then be unmounted.

Typographic Conventions

The conventions used in this document are simple. New concepts that are in-

troduced, as well as URLs, are in italicized font. Binaries and package names

are in bold.Structures, ﬁeld names, compile time deﬁnes and variables are in a

constant-width font. At times, when talking about a ﬁeld in a structure, both the

structure and ﬁeld name will be included as page→list,forexample. File names

are in a constant-width font, but include ﬁles have angle brackets around them like

<linux/mm.h> and may be found in the include/ directory of the kernel source.

Acknowledgments

The compilation of this book was not a trivial task. This book was researched and

developed in the open, and I would be remiss not to mention some of the people

who helped me at various intervals. If there is anyone I missed, I apologize now.

First, I would like to thank John O’Gorman, who tragically passed away while

the material for this book was being researched. His experience and guidance largely

inspired the format and quality of this book.

Second, I would like to thank Mark L. Taub from Prentice Hall PTR for giving

me the opportunity to publish this book. It has been a rewarding experience and

Preface xvii

made trawling through all the code worthwhile. Massive thanks go to my reviewers,

who provided clear and detailed feedback long after I thought I had ﬁnished writing.

Finally, on the publisher’s front, I would like to thank Bruce Perens for allowing me

to publish in the Bruce Perens’ Open Source Series (http://www.perens.com/Books).

With the technical research, a number of people provided invaluable insight.

Abhishek Nayani was a source of encouragement and enthusiasm early in the re-

search. Ingo Oeser kindly provided invaluable assistance early on with a detailed

explanation of how data is copied from userspace to kernel space, and he included

some valuable historical context. He also kindly oﬀered to help me if I felt I ever got

lost in the twisty maze of kernel code. Scott Kaplan made numerous corrections to

anumber of systems from noncontiguous memory allocation to page replacement

policy. Jonathon Corbet provided the most detailed account of the history of kernel

development with the kernel page he writes for Linux Weekly News. Zack Brown,

the chief behind Kernel Traﬃc, is the sole reason I did not drown in kernel-related

mail. IBM, as part of the Equinox Project, provided an xSeries 350, which was in-

valuable for running my own test kernels on machines larger than those I previously

had access to. Late in the game, Jeﬀrey Haran found the few remaining technical

corrections and more of the ever-present grammar errors. Most importantly, I’m

grateful for his enlightenment on some PPC issues. Finally, Patrick Healy was cru-

cial to ensuring that this book was consistent and approachable to people who are

familiar with, but not experts on, Linux or memory management.

Anumber of people helped with smaller technical issues and general inconsisten-

cies where material was not covered in suﬃcient depth. They are Muli Ben-Yehuda,

Parag Sharma, Matthew Dobson, Roger Luethi, Brian Lowe and Scott Crosby. All

of them sent corrections and queries on diﬀerent parts of the document, which

ensured that too much prior knowledge was not assumed.

Carl Spalletta sent a number of queries and corrections to every aspect of the

book in its earlier online form. Steve Greenland sent a large number of grammar

corrections. Philipp Marek went above and beyond being helpful by sending more

than 90 separate corrections and queries on various aspects. Long after I thought

Iwas ﬁnished, Aris Sotiropoulos sent a large number of small corrections and sug-

gestions. The last person, whose name I cannot remember, but is an editor for a

magazine, sent me more than 140 corrections to an early version. You know who

you are. Thanks.

Eleven people sent a few corrections. Though small, they were still missed

by several of myownchecks. They are Marek Januszewski, Amit Shah, Adrian

Stanciu, Andy Isaacson, Jean Francois Martinez, Glen Kaukola, Wolfgang Oertl,

Michael Babcock, Kirk True, Chuck Luciano and David Wilson.

On the development of VMRegress, nine people helped me keep it together.

Danny Faught and Paul Larson both sent me a number of bug reports and helped

ensure that VMRegress worked with a variety of diﬀerent kernels. Cliﬀ White, from

the OSDL labs, ensured that VMRegress would have a wider application than my

owntest box. Dave Olien, also associated with the OSDL labs, was responsible for

updating VMRegress to work with 2.5.64 and later kernels. Albert Cahalan sent

all the information I needed to make VMRegress function against later proc utilities.

Finally, Andrew Morton, Rik van Riel and Scott Kaplan all provided insight on the

CHAPTER

Introduction

Linux is a relatively new operating system that has begun to enjoy a lot of attention

from the business, academic and free software worlds. As the operating system

matures, its feature set, capabilities and performance grow, but so, out of necessity

does its size and complexity. Table 1.1 shows the size of the kernel source code in

bytes and lines of code of the mm/ part of the kernel tree. This size does not include

the machine-dependent code or any of the buﬀer management code and does not

even pretend to be an accurate metric for complexity, but it still serves as a small

indicator.

Version Release Date Total Size Size of mm/ Line Count

1.0 March 13, 1992 5.9MiB 96KiB 3,109

1.2.13 February 8, 1995 11MiB 136KiB 4,531

2.0.39 January 9, 2001 35MiB 204KiB 6,792

2.2.22 September 16, 2002 93MiB 292KiB 9,554

2.4.22 August 25, 2003 181MiB 436KiB 15,724

2.6.0-test4 August 22, 2003 261MiB 604KiB 21,714

Table 1.1. Kernel Size as an Indicator of Complexity

Out of habit, open source developers tell new developers with questions to refer

directly to the source with the “polite” acronym RTFS

,orrefer them to the kernel

newbies mailing list (http://www.kernelnewbies.org). With the Linux VM manager,

this used to be a suitable response because the time required to understand the VM

could be measured in weeks. Moreover, the books available devoted enough time

to the memory management chapters to make the relatively small amount of code

easy to navigate.

The books that describe the operating system such as Understanding the Linux

Kernel [BC00] [BC03] tend to cover the entire kernel rather than one topic with the

notable exception of device drivers [RC01]. These books, particularly Understanding

the Linux Kernel,provideinvaluable insight into kernel internals, but they miss the

details that are speciﬁc to the VM and not of general interest. But the book you are

holding details why ZONE

NORMAL is exactly 896MiB and exactly how per-cpu caches

Read The Flaming Source. It doesn’t really stand for Flaming, but children could be reading.

2 Introduction Chapter 1

are implemented. Other aspects of the VM, such as the boot memory allocator and

the VM ﬁlesystem, which are not of general kernel interest, are also covered in this

book.

Increasingly, to get a comprehensive view on how the kernel functions, one is

required to read through the source code line by line. This book tackles the VM

speciﬁcally so that this investment of time to understand the kernel functions will

be measured in weeks and not months. The details that are missed by the main

part of the book are caught by the code commentary.

In this chapter, there will be an informal introduction to the basics of acquiring

information on an open source project and some methods for managing, browsing

and comprehending the code. If you do not intend to be reading the actual source,

you may skip to Chapter 2.

1.1 Getting Started

One of the largest initial obstacles to understanding code is deciding where to start

and how to easily manage, browse and get an overview of the overall code structure.

If requested on mailing lists, people will provide some suggestions on how to proceed,

but a comprehensive methodology is rarely oﬀered aside from suggestions to keep

reading the source until it makes sense. The following sections introduce some useful

rules of thumb for open source code comprehension and speciﬁc guidelines for how

the rules may be applied to the kernel.

1.1.1 Conﬁguration and Building

With any open source project, the ﬁrst step is to download the source and read

the installation documentation. By convention, the source will have a README or

INSTALL ﬁle at the top level of the source tree [FF02]. In fact, some automated

build tools such as automake require the install ﬁle to exist. These ﬁles contain

instructions for conﬁguring and installing the package or give a reference to where

more information may be found. Linux is no exception because it includes a README

that describes how the kernel may be conﬁgured and built.

The second step is to build the software. In earlier days, the requirement for

many projects was to edit the Makefile by hand, but this is rarely the case now.

Free software usually uses at least autoconf

to automate testing of the build

environment and automake

to simplify the creation of Makefiles, so building is

often as simple as:

mel@joshua: project $ ./configure && make

Some older projects, such as the Linux kernel, use their own conﬁguration tools,

and some large projects such as the Apache Web server have numerous conﬁguration

options, but usually the conﬁgure script is the starting point. In the case of the

http://www.gnu.org/software/autoconf/

http://www.gnu.org/software/automake/

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