Hello, I am not a kernel developer but I think I can answer this. This is how I understand it: Short version: -next is a very special Linux branch managed by Linus Torvalds that contains changes from all subsystem that are considered good candidate for the next release, but people want to see some more testing or "maturation". -mm is one of the many Linux subsysitems (Memory Management). Less short version: The -next tree is a branch of Linux managed by Linus Torvalds containing "experimental" or "not mature enough" changes. When Linus is convinced that a change is good, it will "graduate" it by merging that change into a Linux release candidate (-rc). The time a change sit in -next before graduating to -rc is variable and depends on the change itself; something obviusly correct will graduate fast, while some other change might need a long -next time or could even never graduate. The act of deciding what graduates and what doesn't take place in a point in time known as the "merge window". Better said, a given version of -rc has a well specified deadline for things to get into it; if a change doesn't graduate fast enough, it slips into the next window. So, after this illustration, you might thing of the possibility of some changes rotting in -next forever. Can you imagine after decades of kernel life what sort of mess this "limbo" branch could become? That's why periodically -next is "nuked" and rebuilt from all changes not yet graduated yet but are still worth of keeping around. That's the occasion to do some cleanup. This process of burning and rebuilding -next is mostly automated; one useful tool is "git rerere" [0][1] which let you record conflict fixes you did in past merges. This workflow is sometimes called "-next branch workflow" and is explained in the gitworkflows(7) man page [2]. I made a few pictures to explain it graphically in a post[3] of my blog. linux-mm, in the other hand, is a branch that contains all Memory Management (MM) related changes, as you can see in the MAINTAINERS file[4]. So, about the message you linked: "Most of the -mm tree is already in the linux-next tree" means that at that point in time, October 2013, -next included most of -mm. But the message doesn't say two "obvious" but important things: 1) the other inclusion is not true! -next wasn't equal to -mm, but surely much larger (think of all changes awaiting graduation from the other subsystems) 2) given the dynamic nature of -next, which is periodically destroyed and rebuilt, that statement only held only until the following -next rebuild (i.e. the beginning of the new development cycle). Please take what I say with a grain of salt, as I never participated to the kernel development and I am only an external observer. Consider what I write here as an explaination attempt, and I invite all people who know better to correct it. Cheers, Giovanni [0] https://git-scm.com/docs/git-rerere [1] https://git-scm.com/blog/2010/03/08/rerere.html [2] https://www.kernel.org/pub/software/scm/git/docs/gitworkflows.html [3] http://www.gghh.name/dibtp/2014/07/25/the-next-branch-workflow-and-criss-cro... [4] https://github.com/torvalds/linux/blob/master/MAINTAINERS#L6955