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.

https://lwn.net/Articles/1098399/
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