[$] Introducing maple trees

Seen from outside, the internals of the Linux kernel appear to be stable,especially in subsystems like the memory-management subsystem. However,from time to time, developers need to replace an internalinterface to solve a longstanding problem. One suchissue is contention on the lock used to protect essentialmemory-management structures, including the page tables and virtual memory areas(VMAs). Liam Howlett and Matthew Wilcox have been developing a newdata structure, called a "maple tree", to replace the data structurescurrently used for VMAs. This potentially big change in internal kernelstructures has been recently postedfor a review in a massive patch set.