Back to a fairly core component of Unix systems: The filesystem. Linux 1.0 supports ten filesystems, and I tried to start at the "easy" end and work my way up...
isofs Read-only is simple, right? Unforunately, there's a fair amount of mess to it, and lots of stuff like Rock Ridge implementation and so on, so it's painful to read at the detailed level (I skipped those details). On the other hand, the structure makes it easy to see the big picture.
ext This looked like a good option next. Not as big as ext2, but somewhere in that ancestry. And... yeah, it's quite readable. It's a fairly traditional Unix-y filesystem, and it's all straightforward and pleasant if you don't worry about races.
If you do worry about races, there's plenty of subtlety. We're back in the early days of Linux, so no SMP or random interruption of your FS operations, but if you do any I/O you might get context-switched to something else trying to operate on the same filesystem. "Truncate" is particularly messy, but the whole thing makes me twitch.
There's nothing enforced by the language - it's all "write your code carefully and hope". And... this somehow seems topical, given the recent discovery of the "Dirty COW" race condition that's left a privilege escalation in Linux for the last 10 years. Sigh - we need to do better.
minix It turns out ext is basically a modification of minix. The differences are relatively minor - ext has nice improvements in that the bitmap of used blocks and inodes is replaced with a free list, block numbers are 32-bit rather than 16-bit and triple indirect blocks exist. So, I'm reading things backwards. The removal of free block/inode bitmaps actually makes ext easier to read, though, so it all worked out ok in the end!
By now, I noticed that all the file systems are following the same stereotyped code structure - to the degree that many of them are clearly forks of the minix filesystem.
xiafs is another modification of minix. I assumed "xia" was an abbreviation for something (cf minix, ext - lots of "x"s), but it's just written by Frank Xia! I can see why xiafs was a bit of a dead end - while it dealt with allocations in zones rather than blocks, it doesn't really add much else, and I can see why we followed the path from ext instead.
sysv is another copy of minix, rather neatly designed to cope with several different variations of the the sysv filesystem used by other Unices. As I say, rather neat. It's interesting to see how these various Unix filesystems a) have similar structures to each other, and b) really are quite simple to understand, with simple code to implement them.
proc After several similar file systems, this was a nice distraction and a good example of semi-abusing the fs infrastructure! I remember it being a really cool feature as I first played with Linux, but the implementation is mad. I noticed a bug in that if CONFIG_INET is disabled, then/proc/net/unix appears as a file but doesn't work due to inconsistent placement of #ifdef-ing.
There's a fair amount of filesystem code, so I'll save the rest for another post.