+pages are freed. The pool is not preallocated. By default, a zpool of type
+zbud is created, but it can be selected at boot time by setting the "zpool"
+attribute, e.g. zswap.zpool=zbud. It can also be changed at runtime using the
+sysfs "zpool" attribute, e.g.
+
+echo zbud > /sys/module/zswap/parameters/zpool
+
+The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
+means the compression ratio will always be 2:1 or worse (because of half-full
+zbud pages). The zsmalloc type zpool has a more complex compressed page
+storage method, and it can achieve greater storage densities. However,
+zsmalloc does not implement compressed page eviction, so once zswap fills it
+cannot evict the oldest page, it can only reject new pages.