[$] The [vx]swap showdown

The kernel's swap layer has undergone some significant changes over the course of the last year and a number of longstanding problems have been addressed. One problem that has not yet been solved in the mainline is the direct tie between slots in the swap cache and space in persistent swap files, which can cause highly inefficient resource use. There are two competing solutions for this problem, neither of which has, as yet, reached readiness for merging; a lengthy discussion on possible paths forward shows ongoing disagreement over the best path forward.