So, have you found a bug you want to fix? Want to implement a feature from the TODO list? Got a new feature to implement? Hacking the code couldn't be easier. Just edit your copy of the sources. No need to copy files to .orig or anything. CVS has copies of the originals.
When you have your code in a working state and have tested as best you can with the hardware you have, generate a patch against the current sources in the CVS repository.
# cvs update
# cvs diff > patchfile
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Mail the patch to the linux-lvm or dm-devel list (Section C.1) with a description of what changes or additions you implemented.