[$] Modernizing swapping: virtual swap spaces

The kernel's unloved but performance-critical swapping subsystem has beenundergoing multiple rounds of improvement in recent times. Recent articleshave described the addition of the swaptable as a new way of representing the state of the swap cache, and the removal of the swap map as the way oftracking swap space. Work in this area is not done, though; this series fromNhat Pham addresses a number of swap-related problems by replacing thenew swap table structures with a single, virtual swap space.