Accessing on-disk datastructures from a module

[Martin DeMello]

I would like to maintain some filesystem metadata as an on-disk
datastructure that can be read and written to from within the
filesystem module. I know file access from within the kernel is a bad
idea; what is the recommended alternative for this?

I'm thinking of reserving a bunch of blocks at the start of the
partition and accessing them directly - is this the right approach?
I'm not too sure how to do that either - I figured the way that ext3
maintains its journal would provide a hint, but I can find no
documentation on this, and the code is hard to trace through for this
one specific thing.

martin