So, I was recently talking to a colleague of mine who has spent a good number of years in the defense industry. As he was reminiscing the good old days, the conversation gradually drifted to a more interesting discussion about a guy who was a software developer: It was the cold war era and there was this critical monitoring system (with some version of MULTICS running on it) that needed an important kernel update. And the catch: the system can't be switched off even for a second and (probably) it was a core update or may be LKMs didn't exist then :) So, our smart guy did something that no one had ever done in those times. He built a system with an exactly similar configuration and made whatever changes needed to the kernel. Then, he compiled it and came up with a binary diff between the changed kernel and the original one. The next step: Yes, you guessed it, he identified sections of the running kernel where there have been changes and set the non-executable bits on them (in the critical system). After that, it was just a matter of applying the patch. And there you are! the system stayed on as it always was :) PS: I am really not sure how possible it is with today's OSes but I find the whole idea quite fascinating.