[$] How to get rid of mmap_sem

The mmap_sem lock used in the memory-management subsystem has beena known scalability problem for years, but it has proved difficult toremove. During a session in the memory-management track of the 2019 LinuxStorage, Filesystem, and Memory-Management Summit, Laurent Dufour andMatthew Wilcox discussed a possible solution: replacing the red-black treecurrently used to track virtual memory areas (VMAs) with a new datastructure called a "maple tree".