> Will it get pushed out when the file system fills up?

No. vm_object_cache_trim() will purge it when there are more than
`vm_cache_max' (currently set to 100) cached objects.

> I'm creating a temporary file in /tmp this way.  /tmp is
> an MFS file system and has little space so it's important
> that I can reuse it.
> 
There's no easy solution, perhaps we should have a hint to pass in
mmap(2)'s `flag' parameter. You might be able to work around it by
mapping the file with PROT_WRITE on, if your application can stand it,
which will prevent the VM object from entering the cache.