All memory is conceptually backed by some file. Under memory pressure, the kernel (and this mechanism is the same on any modern kernel) frees memory by writing

All memory is conceptually backed by some file. Under memory pressure, the kernel (and this mechanism is the same on any modern kernel) frees memory by writing pages to their backing storage, then discarding them from RAM. There nothing really special about anonymous memory except that it's backed by swap instead of by a named file on some filesystem. On a system with swap disabled, the backing store is still conceptually swap, but since the swap doesn't exist, pages backed by that imaginary swap can't be evicted from RAM. Pages backed by other backing stores can certainly be evicted from RAM, however, and that's how you "swap" on a swapless system. Note that executable code is almost all mapped so that it can "swap" in this way.