Hah, this is amazing! This reminds me of how I used to (and still do, sometimes) read third-party code.

For an OS class in college, we had to modify fork (and re-build the kernel) to track how many times a particular process had been forked (and probably some other statistics I'm forgetting at the moment).

I remember going through a very similar process for the first time - injecting white space below chunks of code, writing out my own comments, and then using that to figure out how to modify fork. Looking at the author's fork.c comments gave me a feeling of nostalgia.

The useful part of course is going through yourself and writing your own comments, but it can be really helpful to start with something like this (and then write your own version of the comments).