Linux Device Drivers, Second Edition

Backward Compatibility

The demand-loading capability was entirely reimplemented in the 2.1 development series. Fortunately, very few modules need to be aware of the change in any way. For completeness, however, we will describe the old implementation here.

In the 2.0 days, demand loading was handled by a separate, user-space daemon process called kerneld. This process connected into the kernel via a special interface and received module load (and unload) requests as they were generated by kernel code. There were numerous disadvantages to this scheme, including the fact that no modules could be loaded until the system initialization process had gotten far enough to start kerneld.

The request_module function, however, remained unchanged, as did all aspects of the modules themselves. It was, however, necessary to include <linux/kerneld.h> instead of <linux/kmod.h>.

Symbol versioning in the 2.0 kernel did not use the smp_ prefix on SMP systems. As a result, insmod would happily load an SMP module into a uniprocessor kernel, or vice versa. The usual result of such a mismatch was extreme chaos.

The ability to run user-mode helper programs and the intermodule communication mechanism did not exist until Linux 2.4.