内核基础

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最新推荐文章于 2022-08-13 12:58:32 发布

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本文链接：https://blog.youkuaiyun.com/weixin_36888575/article/details/98641163

Linux kernel

单内核体系设计、但充分借鉴了微内核设计体系的优点，为内核引入模块化机制。

内核组成部分：
1. kernel: 内核核心，一般为bzImage，通常在/boot目录下，名称为vmlinuz-VERSION-RELEASE；
2. kernel object: 内核对象，一般放置于/lib/modules/VERSION-RELEASE/，内核启动并不会加载所有的内核文件，而且模块之间有相互依赖关系，这个目录下通常维护多个模块，模块名称到文件路径映射以及多个模块之间的依赖关系等等。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# ls -l /lib/modules/2.6.32-642.el6.x86_64/

total 3860

lrwxrwxrwx. 1 root root 46 Jan 29 09:35 build -> ../../../usr/src/kernels/2.6.32-642.el6.x86_64

drwxr-xr-x. 2 root root 4096 May 10 2016 extra

drwxr-xr-x. 11 root root 4096 Jan 29 09:35 kernel

-rw-r–r–. 1 root root 651373 Jan 29 09:36 modules.alias

-rw-r–r–. 1 root root 618220 Jan 29 09:36 modules.alias.bin

-rw-r–r–. 1 root root 1413 May 10 2016 modules.block

-rw-r–r–. 1 root root 69 Jan 29 09:36 modules.ccwmap

-rw-r–r–. 1 root root 211745 Jan 29 09:36 modules.dep

-rw-r–r–. 1 root root 305708 Jan 29 09:36 modules.dep.bin

-rw-r–r–. 1 root root 78 May 10 2016 modules.drm

-rw-r–r–. 1 root root 665 Jan 29 09:36 modules.ieee1394map

-rw-r–r–. 1 root root 218 Jan 29 09:36 modules.inputmap

-rw-r–r–. 1 root root 1313 Jan 29 09:36 modules.isapnpmap

-rw-r–r–. 1 root root 31 May 10 2016 modules.modesetting

-rw-r–r–. 1 root root 2052 May 10 2016 modules.networking

-rw-r–r–. 1 root root 74 Jan 29 09:36 modules.ofmap

-rw-r–r–. 1 root root 78070 May 10 2016 modules.order

-rw-r–r–. 1 root root 514249 Jan 29 09:36 modules.pcimap

-rw-r–r–. 1 root root 6259 Jan 29 09:36 modules.seriomap

-rw-r–r–. 1 root root 165 Jan 29 09:36 modules.softdep

-rw-r–r–. 1 root root 267597 Jan 29 09:36 modules.symbols

-rw-r–r–. 1 root root 335219 Jan 29 09:36 modules.symbols.bin

-rw-r–r–. 1 root root 881297 Jan 29 09:36 modules.usbmap

lrwxrwxrwx. 1 root root 5 Jan 29 09:35 source -> build

drwxr-xr-x. 2 root root 4096 May 10 2016 updates

drwxr-xr-x. 2 root root 4096 Jan 29 09:35 vdso

drwxr-xr-x. 2 root root 4096 May 10 2016 weak-updates

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[ ]: N 不编译这个功能，不支持
[M]: M 编译成模块，需要时动态加载，需要的时候可以加载，不需要可以卸载，节约内存资源。
[*]: Y 编译核心组成部分，需要直接有不需要加载等待时间。

辅助文件：ramdisk
1. CentOS 6: initramfs，工具程序：mkinitrd, dracut
2. CentOS 5: initrd, 工具程序：mkinitrd

上面是静态视角。

运行中的内核：
1. uname命令：uname – print system information

uname [OPTION]…

-n: 显示节点名称；

[root@centos68 ~]# uname -n

centos68.zmjchina.cn

-r: 显示VERSION-RELEASE;

[root@centos68 ~]# uname -r

2.6.32-642.el6.x86_64

–s：显示操作系统名称

[root@centos68 ~]# uname -s

Linux

–v：版本号

[root@centos68 ~]# uname -v

#1 SMP Tue May 10 17:27:01 UTC 2016
–m：硬件平台 –p：处理器 –i ：硬件平台他们都是统一的。

[root@centos68 ~]# uname -m

x86_64

模块：
1. lsmod命令：

显示由核心已经装载的内核模块

显示的内容来自于: /proc/modules文件

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# lsmod

Module Size Used by

ipt_REJECT 2351 2

nf_conntrack_ipv4 9186 2

nf_defrag_ipv4 1483 1 nf_conntrack_ipv4

iptable_filter 2793 1

ip_tables 17831 1 iptable_filter

ip6t_REJECT 4340 2

nf_conntrack_ipv6 7985 3

nf_defrag_ipv6 26468 1 nf_conntrack_ipv6

xt_state 1492 5

nf_conntrack 79537 3 nf_conntrack_ipv4,nf_conntrack_ipv6,xt_state

ip6table_filter 2889 1

ip6_tables 18732 1 ip6table_filter

ipv6 336282 141 ip6t_REJECT,nf_conntrack_ipv6,nf_defrag_ipv6

ppdev 8217 0

parport_pc 22658 0

parport 36209 2 ppdev,parport_pc

snd_seq_midi 6423 0

snd_seq_midi_event 7205 1 snd_seq_midi

snd_ens1371 21523 0

snd_rawmidi 24633 2 snd_seq_midi,snd_ens1371

snd_ac97_codec 124967 1 snd_ens1371

ac97_bus 1452 1 snd_ac97_codec

snd_seq 60769 2 snd_seq_midi,snd_seq_midi_event

snd_seq_device 7277 3 snd_seq_midi,snd_rawmidi,snd_seq

snd_pcm 93697 2 snd_ens1371,snd_ac97_codec

snd_timer 22747 2 snd_seq,snd_pcm

snd 73783 7 snd_ens1371,snd_rawmidi,snd_ac97_codec,snd_seq,snd_seq_device,snd_pcm,snd_timer

soundcore 7958 1 snd

snd_page_alloc 8856 1 snd_pcm

e1000 134863 0

vmware_balloon 7199 0

i2c_piix4 11232 0

sg 29318 0

shpchp 29130 0

ext4 379655 2

jbd2 93252 1 ext4

mbcache 8193 1 ext4

sd_mod 37158 3

crc_t10dif 1209 1 sd_mod

sr_mod 15049 0

cdrom 39085 1 sr_mod

mptspi 16411 2

mptscsih 36638 1 mptspi

mptbase 93615 2 mptspi,mptscsih

scsi_transport_spi 25447 1 mptspi

pata_acpi 3701 0

ata_generic 3837 0

ata_piix 24409 0

vmwgfx 226420 1

ttm 89408 1 vmwgfx

drm_kms_helper 127219 1 vmwgfx

drm 362893 4 vmwgfx,ttm,drm_kms_helper

i2c_core 29132 3 i2c_piix4,drm_kms_helper,drm

dm_mirror 14864 0

dm_region_hash 12085 1 dm_mirror

dm_log 9930 2 dm_mirror,dm_region_hash

dm_mod 102467 8 dm_mirror,dm_log

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

modinfo命令：

显示模块的详细描述信息

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# modinfo xfs

filename: /lib/modules/2.6.32-642.el6.x86_64/kernel/fs/xfs/xfs.ko

license: GPL

description: SGI XFS with ACLs, security attributes, large block/inode numbers, no debug enabled

author: Silicon Graphics, Inc.

srcversion: 67725EF8353DC29370566C8

depends: exportfs

vermagic: 2.6.32-642.el6.x86_64 SMP mod_unload modversions

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# modinfo xfs -k 2.6.32-642.el6.x86_64

filename: /lib/modules/2.6.32-642.el6.x86_64/kernel/fs/xfs/xfs.ko

license: GPL

description: SGI XFS with ACLs, security attributes, large block/inode numbers, no debug enabled

author: Silicon Graphics, Inc.

srcversion: 67725EF8353DC29370566C8

depends: exportfs

vermagic: 2.6.32-642.el6.x86_64 SMP mod_unload modversions

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

modinfo [ -k kernel ] [ modulename|filename… ]

-n: 只显示模块文件路径

-p: 显示模块参数

-a: author

-d: description

-l: license

lsmod 只想显示当前装载的模块，modinfo是查询数据库来获取模块信息的。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# lsmod | grep xfs

[root@centos68 ~]# modinfo xfs

filename: /lib/modules/2.6.32-642.el6.x86_64/kernel/fs/xfs/xfs.ko

license: GPL

description: SGI XFS with ACLs, security attributes, large block/inode numbers, no debug enabled

author: Silicon Graphics, Inc.

srcversion: 67725EF8353DC29370566C8

depends: exportfs

vermagic: 2.6.32-642.el6.x86_64 SMP mod_unload modversions

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

modprobe命令：

装载或卸载内核模块

modprobe [ -C config-file ] [ modulename ] [ module parame-ters… ]

配置文件：/etc/modprobe.conf, /etc/modprobe.d/*.conf

modprobe [ -r ] modulename..

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# modinfo xfs

filename: /lib/modules/2.6.32-642.el6.x86_64/kernel/fs/xfs/xfs.ko

license: GPL

description: SGI XFS with ACLs, security attributes, large block/inode numbers, no debug enabled

author: Silicon Graphics, Inc.

srcversion: 67725EF8353DC29370566C8

depends: exportfs

vermagic: 2.6.32-642.el6.x86_64 SMP mod_unload modversions

[root@centos68 ~]# lsmod | grep xfs

[root@centos68 ~]# modprobe xfs

[root@centos68 ~]# lsmod | grep xfs

xfs 1135329 0

exportfs 4236 1 xfs

[root@centos68 ~]# modprobe -r xfs

[root@centos68 ~]# lsmod | grep xfs

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++.

depmod命令：

内核模块依赖关系文件及系统信息映射文件的生成工具；

depmod – program to generate modules.dep and map files.

modules.dep—–>编译成二进制文件——-à modules.dep.bin

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# ls /lib/modules/2.6.32-642.el6.x86_64/modules.dep* -l

-rw-r–r–. 1 root root 211745 Jan 29 09:36 /lib/modules/2.6.32-642.el6.x86_64/modules.dep

-rw-r–r–. 1 root root 305708 Jan 29 09:36 /lib/modules/2.6.32-642.el6.x86_64/modules.dep.bin

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

内核所用到的每个库文件彼此间的映射关系。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# ls -l /boot/System.map-2.6.32-642.el6.x86_64

-rw-r–r–. 1 root root 2615003 May 10 2016 /boot/System.map-2.6.32-642.el6.x86_64

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

装载或卸载内核模块：
1. insmod命令：
  
  insmod [ filename ] [ module options… ] 跟modprobe不同的是指定哪个模块装载哪个模块不会解决依赖关系。必须指明路径。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# insmod xfs

insmod: can’t read ‘xfs’: No such file or directory

[root@centos68 ~]# modprobe xfs

[root@centos68 ~]# lsmod | grep xfs

xfs 1135329 0

exportfs 4236 1 xfs

[root@centos68 ~]# lsmod | grep xfs

[root@centos68 ~]# modinfo xfs

filename: /lib/modules/2.6.32-642.el6.x86_64/kernel/fs/xfs/xfs.ko

license: GPL

description: SGI XFS with ACLs, security attributes, large block/inode numbers, no debug enabled

author: Silicon Graphics, Inc.

srcversion: 67725EF8353DC29370566C8

depends: exportfs

vermagic: 2.6.32-642.el6.x86_64 SMP mod_unload modversions

[root@centos68 ~]# insmod /lib/modules/2.6.32-642.el6.x86_64/kernel/fs/xfs/xfs.ko

[root@centos68 ~]# lsmod | grep xfs

xfs 1135329 0

exportfs 4236 1 xfs

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

手动加载btrfs

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# lsmod | grep btrfs

[root@centos68 ~]# modinfo btrfs

filename: /lib/modules/2.6.32-642.el6.x86_64/kernel/fs/btrfs/btrfs.ko

license: GPL

alias: devname:btrfs-control

alias: char-major-10-234

srcversion: B412C18B0F5BF7F1B3C941A

depends: libcrc32c,zlib_deflate,lzo_compress,lzo_decompress

vermagic: 2.6.32-642.el6.x86_64 SMP mod_unload modversions

[root@centos68 ~]# insmod $(modinfo -n btrfs)

insmod: error inserting ‘/lib/modules/2.6.32-642.el6.x86_64/kernel/fs/btrfs/btrfs.ko’: -1 Unknown symbol in module

[root@centos68 ~]# lsmod | grep libcrc32c

[root@centos68 ~]# lsmod | grep zlib_deflate

[root@centos68 ~]# lsmod | grep lzo_compress

[root@centos68 ~]# lsmod | grep lzo_decompress

[root@centos68 ~]# insmod $(modinfo -n libcrc32c)

[root@centos68 ~]# insmod $(modinfo -n zlib_deflate)

[root@centos68 ~]# insmod $(modinfo -n lzo_compress)

[root@centos68 ~]# insmod $(modinfo -n lzo_decompress)

[root@centos68 ~]# insmod $(modinfo -n btrfs)

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

rmmod命令：

rmmod [ modulename ]

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# modprobe xfs

[root@centos68 ~]# lsmod | grep xfs

xfs 1135329 0

exportfs 4236 1 xfs

[root@centos68 ~]# rmmod xfs

[root@centos68 ~]# lsmod | grep xfs

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Modprobe直接卸载关联的模块，而rmmod不会。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# rmmod btrfs

[root@centos68 ~]# lsmod | grep lzo_decompress

lzo_decompress 2343 0

[root@centos68 ~]# lsmod | grep btrfs

[root@centos68 ~]# modprobe -r btrfs

[root@centos68 ~]# lsmod | grep btrfs

[root@centos68 ~]# lsmod | grep lzo_decompress

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

内核把自己内部状态信息及统计信息，以及可配置参数通过proc伪文件系统加以输出；

数字为每一个进程的状态信息。非数字目录或者目录：内核自己状态信息、可配置参数接口、统计信息。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# sysctl -a | wc -l

724

[root@centos68 ~]# ls /proc/

1 1256 1475 15 19 2125 25 270 35 45 50 83 buddyinfo diskstats interrupts key-users meminfo net slabinfo timer_list zoneinfo

10 1278 1488 1507 1968 2191 250 28 36 46 554 9 bus dma iomem kmsg misc pagetypeinfo softirqs timer_stats

11 13 1490 1511 2 22 253 29 37 467 6 916 cgroups driver ioports kpagecount modules partitions stat tty

114 1300 1492 16 20 2210 26 3 38 468 693 917 cmdline execdomains irq kpageflags mounts sched_debug swaps uptime

1174 14 1494 17 21 2255 268 30 399 48 7 956 cpuinfo fb kallsyms loadavg mpt schedstat sys version

1199 1461 1496 18 2102 23 269 33 4 49 8 acpi crypto filesystems kcore locks mtd scsi sysrq-trigger vmallocinfo

12 1471 1498 1845 2107 24 27 34 401 5 82 asound devices fs keys mdstat mtrr self sysvipc vmstat

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

参数：
1. 只读：输出信息
2. 可写：可接受用户指定”新值”来实现对内核某功能或特性的配置

可写的一般都在/proc/sys下面，sysctl设置控制这个目录下的诸多参数。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# ls -l /proc/sys/

total 0

dr-xr-xr-x 0 root root 0 Jan 29 15:45 abi

dr-xr-xr-x 0 root root 0 Jan 29 11:50 crypto

dr-xr-xr-x 0 root root 0 Jan 29 15:45 debug

dr-xr-xr-x 0 root root 0 Jan 29 15:45 dev

dr-xr-xr-x 0 root root 0 Jan 29 11:50 fs

dr-xr-xr-x 0 root root 0 Jan 29 11:50 kernel

dr-xr-xr-x 0 root root 0 Jan 29 11:50 net

dr-xr-xr-x 0 root root 0 Jan 29 11:50 vm

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

sysctl命令用于查看或设定此目录中诸多参数；

sysctl -w path.to.parameter=VALUE

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# find /proc/sys/ -name hostname

/proc/sys/kernel/hostname

[root@centos68 ~]# hostname

centos6868

[root@centos68 ~]# sysctl -w kernel.hostname=centos68.zmjchina.cn

kernel.hostname = centos68.zmjchina.cn

[root@centos68 ~]# hostname

centos68.zmjchina.cn

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

echo命令通过重定向的方式也可以修改大多数参数的值；

echo “VALUE” > /proc/sys/path/to/parameter

~]# echo “www.magedu.com” > /proc/sys/kernel/hostname

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# hostname

centos6868

[root@centos68 ~]# echo “centos68.zmjchina.cn” > /proc/sys/kernel/hostname

[root@centos68 ~]# hostname

centos68.zmjchina.cn

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

sysctl命令：

默认配置文件：/etc/sysctl.conf

设置某参数

sysctl -w parameter=VALUE

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# find /proc/sys/ -name hostname

/proc/sys/kernel/hostname

[root@centos68 ~]# hostname

centos6868

[root@centos68 ~]# sysctl -w kernel.hostname=centos68.zmjchina.cn

kernel.hostname = centos68.zmjchina.cn

[root@centos68 ~]# hostname

centos68.zmjchina.cn

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

通过读取配置文件设置参数

sysctl -p [/path/to/conf_file]

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# sysctl net.ipv4.ip_forward

net.ipv4.ip_forward = 0

[root@centos68 ~]# cat /etc/sysctl.conf | grep “^net.ipv4.ip_forward”

net.ipv4.ip_forward = o

[root@centos68 ~]# vim /etc/sysctl.conf

[root@centos68 ~]# cat /etc/sysctl.conf | grep “^net.ipv4.ip_forward”

net.ipv4.ip_forward = 1

[root@centos68 ~]# sysctl net.ipv4.ip_forward

net.ipv4.ip_forward = 0

[root@centos68 ~]# sysctl -p /etc/sysctl.conf

net.ipv4.ip_forward = 1

net.ipv4.conf.default.rp_filter = 1

net.ipv4.conf.default.accept_source_route = 0

kernel.sysrq = 0

kernel.core_uses_pid = 1

net.ipv4.tcp_syncookies = 1

kernel.msgmnb = 65536

kernel.msgmax = 65536

kernel.shmmax = 68719476736

kernel.shmall = 4294967296

[root@centos68 ~]# sysctl net.ipv4.ip_forward

net.ipv4.ip_forward = 1

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

查询有多少参数可用 –a：

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# sysctl -a | wc -l

724

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

常用的几个参数：
1. net.ipv4.ip_forward

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# cat /proc/sys/net/ipv4/ip_forward

[root@centos68 ~]# sysctl net.ipv4.ip_forward

net.ipv4.ip_forward = 0

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

vm.drop_caches

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# free

total used free shared buffers cached

Mem: 1012080 805208 206872 264 52124 562464

-/+ buffers/cache: 190620 821460

Swap: 2031612 0 2031612

[root@centos68 ~]# ls /proc/sys/vm/drop_caches -l

-rw-r–r– 1 root root 0 Jan 29 15:45 /proc/sys/vm/drop_caches

[root@centos68 ~]# sysctl -w vm.drop_caches=1

vm.drop_caches = 1

[root@centos68 ~]# free

total used free shared buffers cached

Mem: 1012080 184624 827456 264 188 10608

-/+ buffers/cache: 173828 838252

Swap: 2031612 0 2031612

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

kernel.hostname

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[root@centos68 ~]# find /proc/sys/ -name hostname

/proc/sys/kernel/hostname

[root@centos68 ~]# hostname

centos6868

[root@centos68 ~]# sysctl -w kernel.hostname=centos68.zmjchina.cn

kernel.hostname = centos68.zmjchina.cn

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

/sys目录：
1. sysfs：输出内核识别出的各硬件设备的相关属性信息，也有内核对硬件特性的设定信息；有些参数是可以修改的，用于调整硬件工作特性。我们都知道dev是输出设备文件的，那么我们系统又如何知道有哪些设备又如何创建设备文件的呢？内核启动的是时候通过探测各硬件的io port 端口上有哪些设备连接进来，并通过获取这些硬件自己的工作电压所输出的这些硬件信息，才能知道这是内核。只有用户空间访问硬件设备才会使用设备文件，内核自身不需要。2.4以前设备文件都是事先创建好的，无论是否有都会有。（有进20000个，实际使用就几十个）。内核无论探测多少设备都不会自动为它创建设备文件，dev目录下的设备文件，2.4后让内核启动后探测硬件设备，探测完以后该启动的启动，但是当用户空需要用到此前探测到的设备文件，用内生机制触发探再测一遍然后输出到sys目录。输出保存所有的硬件设备。
2. 用户空间udev通过此路径下输出的信息动态为各设备创建所需要设备文件；udev是运行用户空间程序；专用工具：udevadmin, hotplug；
3. udev为设备创建设备文件时，会读取其事先定义好的规则文件，一般在/etc/udev/rules.d及/usr/lib/udev/rules.d目录下；