When should I write a Linux kernel module?

Question

Some people want to move code from user space to kernel space in Linux for some reason. A lot of times the reason seems to be that the code should have particularly high priorit

Answer 1

There are very limited reasons to put stuff into the kernel. If you're writing device drivers it's ok. Any standard application: never.

The drawbacks are huge. Debugging gets harder, errors become more frequent and hard to find. You might compromise security and stability. You might have to adapt to kernel changes more frequently. It becomes impossible to port to other UNIX OSs.

The closest I've ever come to the kernel was a custom filesystem (with mysql in the background) and even for that we used FUSE (where the U stands for userspace).