Reading the Leffler book chapter on filesystems, it appears there were
substantial modifications to the filesystem code.  For example, I can't
seem to find bio(), and bmap() appears as ufs_bmap/ufs_bmaparray
instead of the form he describes.  Additionally, I'm not clear how
ffs and ufs differ; they have distinct subdirectories under sys/ufs
but the mount_[uf]fs manpage makes them sound like the same thing.

I assume it has to do with the stackable vfs/vnode/whatever thing.
Does anyone have any references where I can read about how the filesystem
works now?

Also, if I ported ext2fs filesystem driver (that's a big if) from FreeBSD,
it would not go in the development tree because it is GPLed, right?