Learning ext2 fs

[Paulo da Silva <psdasilva@esoterica.pt>]

I am reading the ext2 fs code. One of my purposes
is to save the original data of a file to another file
just before it is changed by write/mmap/whatever.
Because of mmap (any other reasons?) I thought
of doing this at "ext2-writepage" or/and
"ext2-writepages".

Is this the right place?
Is there a lower level where I can read/write blocks
of data from/to hd instead of full pages?

How do I tell the really file data from other data?

I traced these functions but I only got
"ext2-writepages" to be called. "ext2-writepage"
was never called using the programs
I wrote to test this. When is "ext2-writepage" called?

Thanks for any help.
Any readings advice is also welcome.