FYI, the main reason (well, besides history) that XFS uses its own quota code is so that it can have full integration with the transaction subsystem and the on-disk data structures. While it is going to be a lot more code you might want to look into implementing quota accounting in ubifs itself and only use the standard interfaces to report and adminstrate the quota values. That way you might not even have to maintain a separate quota file and could instead maintain usage statistics in the btree nodes, and only summarize them in memory, possibly with a periodical full state dump.
