ext4 Specifications?

Where can I find the ext4 file system specifications?

(Unofficial drafts are fine, so long as they're rea开发者_运维知识库sonably up-to-date; if unavailable, ext3 would be fine too.)

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I've been using ext4 Disk Layout @ kernel.org. Requires careful reading and some extra research on subjects but for the most part it is complete.