[$] Memory-management short topics: page-table sharing and working sets

The kernel's memory-management developers have been busy before and duringthe holidays; the result is a number of patch sets making significantchanges to that subsystem. It is time for a quick look at three of thoseprojects. Two of them aim to increase the sharing of page tables betweenprocesses, while the third takes advantage of the multi-generational LRU to create a betterpicture of what a process's working set actually is.