"Reiser4, the successor to the popular Reiserfs by Hans Reiser,
has been in development for a number of years now and, for almost
as long, Reiser has been pushing for inclusion of the filesystem in
mainline--that is, to make it into the official kernel release.
While reiserfs has been part of the kern since 2.4.1, Reiser4
remains a feature offered only in third party kernel