lsmod, rmmod, modprobe, and modinfo commands
What is Linux Kernel?
The kernel is just a piece of code. Ok, not a bit of code 1000’s of lines. For example, Linux kernel has 20 million lines of code. A kernel is a central part of computer operating system which acts as a mediator between system hardware and applications installed in the system. The Kernel converts program’s instructions to machine understandable language and passes them through device drivers.
What are types of Kernel?
We have two types of Kernels are there.
- Monolithic kernel
- Modular kernel
Before going into difference of kernels we should know what is a kernel module is
What is a Kernel module?
A kernel module is a part of the kernel which can do some work. Say for example Linux Kernel will not support all types of hardware, So programmers will write some kernel modules and distribute them along with their software. Whenever you want to make this hardware to work, we have to load this piece of software into the kernel so that your system understands it. This software is nothing but Kernel module. The modules also referred to as device drivers are specialized pieces of software which power or control access to a peripheral. They are loaded into or unloaded from the running kernel without the need of a reboot.
What is a Monolithic kernel?
A monolithic kernel is where the entire operating system works in kernel space. There is no such space called User space where you can load/remove kernel modules from userspace. In other words, entire device drivers, Filesystem, and IPC will work in kernel space.
What is a Modular kernel?
A modular kernel is where only a particular part of OS is loaded into kernel space. In this way, we will reduce the burden on the system as we can load kernel modules into kernel where and whenever it is required. And the services will run complete isolation from Kernel space.
The /proc/devices file contains a mapping of the device major numbers and device types broadly classified as character and block devices. The kernel gains access to these devices through the corresponding module provided that it has been loaded correctly. Given below is a snippet of the content contained in this file.
[root@linuxnix ~]# head /proc/devices Character devices: 1 mem 4 /dev/vc/0 4 tty 4 ttyS 5 /dev/tty 5 /dev/console 5 /dev/ptmx 7 vcs 10 misc
The number on the left is the primary number of the module which is used internally by the kernel to identify the peripheral type and the names to the right of the number represent the device type.
The /proc/modules file contains a list of currently loaded modules. Here’s a snippet from the file:
[root@linuxnix ~]# head /proc/modules ebtable_filter 12827 0 - Live 0xffffffffa0657000 ebtables 35009 1 ebtable_filter, Live 0xffffffffa0649000 ip6table_filter 12815 0 - Live 0xffffffffa048a000 ip6_tables 26901 1 ip6table_filter, Live 0xffffffffa0641000 iptable_filter 12810 0 - Live 0xffffffffa048f000 vmw_vsock_vmci_transport 30577 0 - Live 0xffffffffa0481000 vsock 34855 3 vmw_vsock_vmci_transport, Live 0xffffffffa0463000 snd_seq_midi 13565 0 - Live 0xffffffffa05f0000 snd_seq_midi_event 14899 1 snd_seq_midi, Live 0xffffffffa063c000 intel_powerclamp 14419 0 - Live 0xffffffffa0631000
The name in the leftmost/first column in the file represents the module name.
We could go to /sys/module/<module_name> directory to view the files that hold the settings for the driver.
Example 1: To view the corresponding files for the SCSI disk driver represented by sd_mod module, I could head to the /sys/module/sd_mod directory.
[root@linuxnix ~]# cd /sys/module/sd_mod [root@linuxnix sd_mod]# ls coresize holders initsize initstate notes refcnt rhelversion sections srcversion taint uevent
How to see module loaded into the kernel?
Example 2: If we open the initstate and rhelversion files they show that the module is loaded indicated by “live” and the OS for which it’s been loaded is 7.3
[root@linuxnix sd_mod]# cat initstate live [root@linuxnix sd_mod]# [root@linuxnix sd_mod]# cat rhelversion 7.3
To manage kernel modules, we use the lsmod and modprobe commands.
We can add specific options to the modules to customize external access. These options can be configured in the /etc/modprobe.d directory.
Example 3: We use the lsmod command to list currently loaded drivers. This command actually obtains its data from the /proc/modules file.
Output(Clipped):
Last login: Tue Oct 24 21:24:04 2017 from 49.180.168.95 root@linuxnix:~# lsmod Module Size Used by ufs 73728 0 msdos 20480 0 xfs 970752 0 ppdev 20480 0 kvm_intel 172032 0 kvm 544768 1 kvm_intel irqbypass 16384 1 kvm crct10dif_pclmul 16384 0 crc32_pclmul 16384 0 ghash_clmulni_intel 16384 0 joydev 20480 0 input_leds 16384 0 serio_raw 16384 0 parport_pc 32768 0 pvpanic 16384 0 parport 49152 2 ppdev,parport_pc ib_iser 49152 0 rdma_cm 49152 1 ib_iser
Example 4: To filter out for a specific module we can use AWK command as shown below.
[root@linuxnix ~]# lsmod | awk ‘NR==1 {print} /^sd_mod/ {print}’
Module Size Used by
sd_mod 46322 3
The size column indicates the amount of memory used by the module in bytes and the used by field indicates the number of modules that are dependent on this module is loaded.
How to load Kernel modules in Linux?
Now I’ll demonstrate how we could load and unload modules using modprobe. For this demonstration, I’ll be using the sr_mod module which is used to access the cd drive.
[root@linuxnix ~]# lsmod | awk 'NR==1 {print} /^sr_mod/ {print}'
Module Size Used by
sr_mod 22416 0I won’t be working with the sd_mod module here because whatever happens we don’t unload sd_mod and probably lose access to the disks attached to the system.
Example 5: We use modprobe with the -r option to unload a module. We can add the -v option for verbose output.
[root@linuxnix ~]# modprobe -rv sr_mod rmmod sr_modrmmod rmmod cdrom
Now if I search for this module in lsmod output or /proc/modules I won’t find it because we’ve just unloaded it.
[root@linuxnix ~]# lsmod | awk 'NR==1 {print} /^sr_mod/ {print}'
Module Size Used by
[root@linuxnix ~]# grep sr_mod /proc/modulesAt this point, we’ve lost access to our cdrom peripheral.
Example 6: To load the module type modporbe followed by the module name. You may add the -v flag for verbose output.
[root@linuxnix ~]# modprobe -v sr_mod insmod /lib/modules/3.10.0-514.el7.x86_64/kernel/drivers/cdrom/cdrom.ko insmod /lib/modules/3.10.0-514.el7.x86_64/kernel/drivers/scsi/sr_mod.ko [root@linuxnix ~]#
With that, the module has been loaded, and we have regained access to the systems’ cdrom.
[root@linuxnix ~]# lsmod | awk 'NR==1 {print} /^sr_mod/ {print}'
Module Size Used by
sr_mod 22416 0
[root@linuxnix ~]# grep sr_mod /proc/modules
sr_mod 22416 0 - Live 0xffffffffa0209000
cdrom 42556 1 sr_mod, Live 0xffffffffa065c000
[root@linuxnix ~]#You may have observed from the above Example 5 demonstrated modprobe command is calling rmmod to unload a module and insmod to load a module. You may use these commands directly as well.
Example 7: We can use the modinfo command to obtain some descriptive information about the module. Given below is the output obtained by running the modinfo command for the sd_mod module.
[root@linuxnix ~]# modinfo sd_mod filename: /lib/modules/3.10.0-514.el7.x86_64/kernel/drivers/scsi/sd_mod.ko alias: scsi:t-0x0e* alias: scsi:t-0x07* alias: scsi:t-0x00* alias: block-major-135-* alias: block-major-134-* alias: block-major-133-* alias: block-major-132-* alias: block-major-131-* alias: block-major-130-* alias: block-major-129-* alias: block-major-128-* alias: block-major-71-* alias: block-major-70-* alias: block-major-69-* alias: block-major-68-* alias: block-major-67-* alias: block-major-66-* alias: block-major-65-* alias: block-major-8-* License: GPL description: SCSI disk (sd) driver author: Eric Youngdale rhelversion: 7.3 srcversion: 3BCC1FE5C7B281E00A15BEF depends: crc-t10dif intree: Y vermagic: 3.10.0-514.el7.x86_64 SMP mod_unload modversions signer: CentOS Linux kernel signing key sig_key: D4:88:63:A7:C1:6F:CC:27:41:23:E6:29:8F:74:F0:57:AF:19:FC:54 sig_hashalgo: sha256
Example 8: Here is another example of using modinfo this time using the dm_mod module which is the device mapper module.
[root@linuxnix ~]# modinfo dm_mod filename: /lib/modules/3.10.0-514.el7.x86_64/kernel/drivers/md/dm-mod.ko license: GPL author: Joe Thornber <dm-devel@redhat.com> description: device-mapper driver alias: devname:mapper/control alias: char-major-10-236 rhelversion: 7.3 srcversion: F452D86551786DA41676AC1 depends: intree: Y vermagic: 3.10.0-514.el7.x86_64 SMP mod_unload modversions signer: CentOS Linux kernel signing key sig_key: D4:88:63:A7:C1:6F:CC:27:41:23:E6:29:8F:74:F0:57:AF:19:FC:54 sig_hashalgo: sha256 parm: reserved_rq_based_ios:Reserved IOs in request-based mempools (uint) parm: use_blk_mq:Use block multiqueue for request-based DM devices (bool) parm: dm_mq_nr_hw_queues:Number of hardware queues for request-based dm-mq devices (uint) parm: dm_mq_queue_depth:Queue depth for request-based dm-mq devices (uint) parm: stats_current_allocated_bytes:Memory currently used by statistics (ulong) parm: major:The major number of the device mapper (uint) parm: reserved_bio_based_ios:Reserved IOs in bio-based mempools (uint) parm: dm_numa_node:NUMA node for DM device memory allocations (int)
The fields that are present in the above output are actually the parameters which can be tweaked while loading the modules.
This concludes our exploration of kernel modules in Linux. I hope this has been an interesting read for you.
Sahil Suri
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