Understanding the Linux Kernel, Second Edition

Linking Modules on Demand

A module can be automatically linked when the functionality it provides is requested and automatically removed afterward.

For instance, suppose that the MS-DOS filesystem has not been linked, either statically or dynamically. If a user tries to mount an MS-DOS filesystem, the mount( ) system call normally fails by returning an error code, since MS-DOS is not included in the file_systems list of registered filesystems. However, if support for automatic linking of modules has been specified when configuring the kernel, Linux makes an attempt to link the MS-DOS module, and then scans the list of registered filesystems again. If the module is successfully linked, the mount( ) system call can continue its execution as if the MS-DOS filesystem were present from the beginning.

The modprobe Program

To automatically link a module, the kernel creates a kernel thread to execute the modprobe external program,^[141] which takes care of possible complications due to module dependencies. The dependencies were discussed earlier: a module may require one or more other modules, and these in turn may require still other modules. For instance, the MS-DOS module requires another module named fat containing some code common to all filesystems based on a File Allocation Table (FAT). Thus, if it is not already present, the fat module must also be automatically linked into the running kernel when the MS-DOS module is requested. Resolving dependencies and finding modules is a type ...