What is a kernel module explain the life cycle of Linux kernel module?

Answer 1

A kernel module is a piece of the kernel that is not built as part of the kernel. This is largely to keep the kernel from having to initialize and run components it does not need.

By far the most common application of the loadable kernel modules is device drivers. If you have an nVidia card you likely won't need, for example, and Intel or AMD/ATI drivers loaded, but you *might* want Nouveau.

This allows the kernel to be smaller and more memory efficient, while still providing all the needed services required for the hardware on your computer.

The "life cycle" is usually something like, in the case of devce drivers.

1. The kernel catalogues all your hardware by probing all the buses.

2. Udev goes through everything the kernel has noticed and, through rulesets, determines what drivers to probe into the kernel and what permissions and nodes need to be created for device files in /dev.

3. The module should, by this point, have been loaded and run, initializing whatever hardware it supervises, and you use your computer. MOST device drivers don't get unloaded. They simply run for the whole time the system is up in case you need them, which in most cases you do. They can be unloaded, but often even after the hardware is disconnected, udev might notice, but still keep the driver loaded and simply adjust /dev to reflect the device removal.

In cases of non-device drivers, it is often more like this:

1. The user or a program with sufficient privileges realizes they need some specific service not built into the kernel. The user will usually issue the command "modprobe" with the module they need then proceed with their work. Software make make some system calls or some API calls to get a module loaded into the kernel for it so it can work, this is often what occurs with userspace filesystems: They immediately request that the fuse driver get loaded.

2. Whatever the user or software need to do gets done.

3. Often the module stays loaded like device drivers, but a user may issue "rmmod" to unload the driver if they are concerned about memory consumption.

Pretty much the only "common" time a module gets unloaded is if a new version of the module is installed. Thus you'd rmmod and modprobe the new module, but most times this use case won't even work (Most modules are updated as part of a kernel upgrade and therefore the kernel would reject the module as an "API mismatch.")

For most hardware this isn't really much of an issue. For video drivers, usually proprietary ones, it is, and you'll have to reboot to get the use of the new driver, one of the only two cases where restarting Linux might be "necessary," the other being a kernel upgrade.