So what's that 298 MB over there?
Wired
Non-pageable memory: cannot be freed until explicitly released by the owner
Userland memory can be wired by mlock(2) (subject to system and per-user limits)
Kernel memory allocators return wired memory
Contents of the ARC and the buffer cache are wired
Some memory is permanently wired and is never freed (e.g., the kernel file itself)