Red Hat Enterprise Linux Diagnostics and Troubleshooting
The running Linux kernel is constructed from feature and driver modules that can be loaded and unloaded on demand. Use of kernel modules keeps the base kernel memory footprint small and ensures that code is loaded into memory only when required.
Most device drivers are built as kernel modules, as are file system drivers, encryption algorithms, and many other kernel features. Modules are found in the /lib/modules/<KERNEL_VERSION>/kernel/<SUBSYSTEM> directory as compressed <KERNEL_MODULE_NAME>.ko.xz object files.
Kernel modules have regular names and aliases. Aliases are used to provide shorter name alternatives, or for standardizing names for management routines to recognize. For example, USB devices create aliases in the form usb:v0123d0001 to display the vendor ID 0123 and device ID 0001 in the device name. Similar letter patterns are standardized for all kernel modules.
The lsmod command lists the currently loaded modules. The output includes the loaded module name, its size in bytes, the number of other modules that currently use this module, and an optional list of those other modules.
[user@host ~]$ lsmod
Module Size Used by
nft_fib_inet 16384 1
nft_fib_ipv4 16384 1 nft_fib_inet
nft_fib_ipv6 16384 1 nft_fib_inet
nft_fib 16384 3 nft_fib_ipv6,nft_fib_ipv4,nft_fib_inet
nft_reject 16384 1 nft_reject_inet
nft_ct 20480 10
nf_tables_set 49152 13
nft_chain_nat 16384 12
nf_nat 45056 1 nft_chain_nat
nfit 65536 0
libnvdimm 192512 1 nfit
kvm_intel 315392 0
kvm 847872 1 kvm_intel
...output omitted...Developers use the insmod and rmmod commands to load and unload modules. However, recommended practice for administrators is to use the modprobe command instead, which is more capable and handles module dependencies. The modprobe command handles options or aliases that are defined in configuration files in /etc/modprobe.d/. To unload a module, use modprobe -r. A module in use cannot be unloaded until components that use that module are exited or detached.
With the -v option, modprobe can display the insmod and rmmod commands in use.
[root@host ~]$modprobe mce-inject -vinsmod /lib/modules/4.18.0-305.el8.x86_64/kernel/arch/x86/kernel/cpu/mce/mce-inject.ko.xz [root@host ~]$modprobe -r mce-inject -vrmmod mce_inject
Note
The modprobe exit codes indicate whether the end result was achieved, rather than whether the action was performed. For example, when loading an already loaded module, modprobe returns true (exit code 0), even though no load action occurred.
Many kernel modules support optional parameters to alter their behavior. To view a module's parameters, use the modinfo -p <MODULE> command. Alternatively, most modules set their parameters within the /sys/module/<MODULE_NAME>/parameters directory, with each parameter as a separate file. If a parameter is modifiable at runtime, the corresponding file is root writable. If a parameter is configurable only at module load time, the corresponding file is read-only.
To view the kvm module's parameters, use modinfo -p. The output includes parameter names, descriptions, and value types, such as integer, Boolean, or string.
[root@host ~]# modinfo -p kvm
tdp_mmu: (bool)
nx_huge_pages: (bool)
nx_huge_pages_recovery_ratio: (uint)
flush_on_reuse: (bool)
ignore_msrs: (bool)
report_ignored_msrs: (bool)
min_timer_period_us: (uint)
kvmclock_periodic_sync: (bool)
tsc_tolerance_ppm: (uint)
lapic_timer_advance_ns: (int)
vector_hashing: (bool)
enable_vmware_backdoor: (bool)
force_emulation_prefix: (bool)
pi_inject_timer: (bint)
halt_poll_ns: (uint)
halt_poll_ns_grow: (uint)
halt_poll_ns_grow_start: (uint)
halt_poll_ns_shrink: (uint)You can view the same kvm module parameters with the /sys/module/kvm/parameters directory.
[root@host ~]# ls -l /sys/module/kvm/parameters/
total 0
-r--r--r--. 1 root root 4096 Sep 24 12:23 enable_vmware_backdoor
-rw-r--r--. 1 root root 4096 Sep 24 12:23 flush_on_reuse
-r--r--r--. 1 root root 4096 Sep 24 12:23 force_emulation_prefix
-rw-r--r--. 1 root root 4096 Sep 24 12:23 halt_poll_ns
-rw-r--r--. 1 root root 4096 Sep 24 12:23 halt_poll_ns_grow
-rw-r--r--. 1 root root 4096 Sep 24 12:23 halt_poll_ns_grow_start
-rw-r--r--. 1 root root 4096 Sep 24 12:23 halt_poll_ns_shrink
-rw-r--r--. 1 root root 4096 Sep 24 12:23 ignore_msrs
-r--r--r--. 1 root root 4096 Sep 24 12:23 kvmclock_periodic_sync
-rw-r--r--. 1 root root 4096 Sep 24 12:23 lapic_timer_advance_ns
-rw-r--r--. 1 root root 4096 Sep 24 12:23 min_timer_period_us
-rw-r--r--. 1 root root 4096 Sep 24 12:23 mmu_audit
-rw-r--r--. 1 root root 4096 Sep 24 12:23 nx_huge_pages
-rw-r--r--. 1 root root 4096 Sep 24 12:23 nx_huge_pages_recovery_ratio
-rw-r--r--. 1 root root 4096 Sep 24 12:23 pi_inject_timer
-rw-r--r--. 1 root root 4096 Sep 24 12:23 report_ignored_msrs
-rw-r--r--. 1 root root 4096 Sep 24 12:23 tdp_mmu
-rw-r--r--. 1 root root 4096 Sep 24 12:23 tsc_tolerance_ppm
-r--r--r--. 1 root root 4096 Sep 24 12:23 vector_hashingDetailed parameter descriptions and values for many kernel modules are found in files in the kernel-doc package, which is not installed by default.
To pass a parameter to a module, add the parameter=value option to the modprobe command when loading the module. This example sets the buffer_kbs parameter to 64 for the st module:
[root@host ~]# modprobe -v st buffer_kbs=64
insmod /lib/modules/3.10.0-327.el7.x86_64/kernel/drivers/scsi/st.ko buffer_kbs=64To set this parameter each time that the module is loaded, use a configuration file in the /etc/modprobe.d/ directory. Module configuration files require that file names end in .conf. Files are parsed in alphanumeric order. Add parameters with an options line.
options st buffer_kbs=64 max_sg_segs=512
Group options into files that are named for their module, such as /etc/modprobe.d/st.conf, or into files that relate to a function, such as /etc/modprobe.d/throughput.conf.
