Monday, February 13, 2012

Module Management in RHEL / CentOS

The Linux kernel allows drivers and features to be compiled as modules rather than as part of the kernel itself. This means that users can often change features in the kernel or add drivers without recompiling, and that the Linux kernel doesn't have to carry a lot of unnecessary baggage. Want to learn how to manage your modules? It's easy to do, just keep reading.

In this tutorial, we'll walk through the steps of seeing what's already loaded in the running kernel, and adding and removing modules from the kernel.

What's Loaded

The first utilities that you'll want to get to know are lsmod and modinfo. Open a terminal and run lsmod. Note that you won't need to use sudo or log in as root just to probe the modules in the system.

You'll see output like this when you use lsmod:

Module                  Size  Used by
parport_pc             18855  0
ppdev                   5030  0
lp                      7462  0
sco                     7209  2
parport                27954  3 parport_pc,ppdev,lp
bridge                 39630  0
stp                     1440  1 bridge
bnep                    9427  2

This shows the modules that are loaded, their size, and whether they're being used by other modules. Take the parport module, for instance. It's being used by several other modules, but what are they? The modinfo utility will tell us — maybe.

Run #modinfo parport, and you'll see something like this:

filename:       /lib/modules/2.6.32-5-amd64/kernel/drivers/parport/parport.ko
license:        GPL
vermagic:       2.6.32-5-amd64 SMP mod_unload modversions

It tells us where the module is found, and its license, and that it has no dependencies. Unfortunately, we really don't know any more than where we started because this module author has chosen not to provide a description of the module. But many modules will have a description and provide some indication what they are used for. Since we got no joy from the parport module, what about trying to find out about one of the modules that depend on it? Let's try modinfo parport_pc. This, at least on my system, produces fairly hefty output, but the relevant part here is the description field which provides:

description: PC-style parallel port driver

So we can surmise that the parport driver has something to do with supporting a Parallel Port. In fact, the parport module is generic support for parallel ports, and parport_pc provides support for parallel ports on x86/x86_64 systems. This isn't something you'll find with modinfo, unfortunately. But when all else fails, check the kernel source under the Documentation directory.

Removing Modules

Modules can be removed using the rmmod utility. The usage is simple, just rmmod modulename. However, if we try to remove the parport module, we get this error:

ERROR: Module parport is in use by parport_pc,ppdev,lp

You can force module removal using rmmod -f, but that's not a good idea, usually. A better way to do it is to use modprobe -r which will automatically look to see what other modules depend on it, and unload those modules as well. If they're in use, then modprobe will refuse to remove them as well, unless you use the -f option with modprobe too.

Installing Modules

What if you have a module you want to load into the kernel? You can do that with insmod or modprobe.

The preferred method is modprobe, because it will also load any modules that the requested module depends on. For instance, if I didn't have the parport module loaded and went to load the lp or parport_pc modules, modprobe would go ahead and load parport as well.

To load a module using modprobe run #modprobe modulename.

Blacklisting Modules

You may on occasion need to "blacklist" a module. Why would you need this feature? Sometimes a module will cause a conflict with another module, is superseded by another module, or is otherwise undesirable.

To blacklist a module, the easiest way to do it (there's usually more than one way to do things...) is to add the module to /etc/modprobe.d/blacklist.conf. For instance, on Debian systems the evbug module is automatically blacklisted because it's not something most users will need. To add a module to the blacklist, just add one line to the blacklist.conf file:

blacklist modulename

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