These are experimental kernels released by Andrew Morton. The -mm tree serves as a sort of proving ground for new features and other experimental patches. Once a patch has proved its worth in -mm for a while Andrew pushes it on to Linus for inclusion in mainline. This branch is in constant flux and contains many experimental features, a lot of debugging patches not appropriate for mainline etc and is the most experimental of the branches described in this document. These kernels are not appropriate for use on systems that are supposed to be stable and they a more risky to run than any +of the other branches