... btw, I rather imagine that Robert has already noticed this, but OS X
(and presumably other BSDen) spells the flag "MAP_ANON" not
"MAP_ANONYMOUS". I also find this rather interesting flag there:
MAP_HASSEMAPHORE Notify the kernel that the region may contain sema-
phores and that special handling may be necessary.
By "semaphore" I suspect they mean "spinlock", so we'd better turn this
flag on where it exists.
regards, tom lane