Architecture

本章以“结构化思维”对Linux OS进行总结。

The Process/Kernel Model

在Understanding.The.Linux.kernel.3^rd.Edition的chapter 1.6.1. The Process/Kernel Model中所描述Process/Kernel Model与上面所描述的architecture相同，这个名称更加精简，便于表达。对于一个linux OS programmer，心中应该牢记这个model。

Interface: system call

本节标题的含义是: system call是process和kernel之间的interface。

Linux采用的是monolithic kernel，这就使得Linux OS的”Process/Kernel Model“更加地简化:

process的所有请求都是通过system call来向kernel发起的

Control

从**控制论**的角度来看，process的执行收scheduler（位于kernel中）的控制，即process受控于kernel。

关于控制论，参见工程software-engineering的Software-design\Control-theory章节。

Kernel space and user space

Kernel space 和 user space 能够将Linux OS进一步进行划分。下面这篇文章还描述了这种划分的意义。

wikipedia User space

A modern computer operating system usually segregates virtual memory into kernel space and user space.[a] Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour.

Kernel space is strictly reserved for running a privileged operating system kernel, kernel extensions, and most device drivers. In contrast, user space is the memory area where application software and some drivers execute.

Overview

The term userland (or user space) refers to all code that runs outside the operating system's kernel.[1] Userland usually refers to the various programs and libraries that the operating system uses to interact with the kernel: software that performs input/output, manipulates file system objects, application software, etc.

NOTE: 下面对user space process的总结非常好，可以看到Linux OS的很多特性都是在此基础上建立起来的。

Each user space process normally runs in its own virtual memory space, and, unless explicitly allowed, cannot access the memory of other processes. This is the basis for memory protection in today's mainstream operating systems, and a building block for privilege separation.

NOTE: separation

A separate user mode can also be used to build efficient virtual machines – see Popek and Goldberg virtualization requirements.

NOTE: virtual machine

With enough privileges, processes can request the kernel to map part of another process's memory space to its own, as is the case for debuggers.

NOTE: debugger

Programs can also request shared memory regions with other processes, although other techniques are also available to allow inter-process communication.

NOTE: shared memory

NOTE: 原文的这张图总结的非常好；它是对Architecture-of-computing-system中的图的扩充。

NOTE: 需要注意的是，systemd是运行在user mode，而不是kernel mode；

Implementation

The most common way of implementing a user mode separate from kernel mode involves operating system protection rings.

NOTE: 后面会讲解Linux kernel的实现