Each swap area has a priority, either high or low. The default priority is low. Within the low-priority areas, newer areas are even lower priority than older areas.
All priorities set with swapflags are high-priority, higher than default.
They may have any non-negative value chosen by the caller. Higher numbers mean higher priority. Swap pages are allocated from areas in priority order, highest priority first. As of Linux 1.3.6, the kernel usually follows these rules, but there are exceptions. On success, zero is returned.