>> How do I stop Linux from doing this? > You can't disable disk caching

While that's true, you can temporarily stop (1) the cache it with "echo 3 >/proc/sys/vm/drop_caches" and enjoy a short and horrible performance degradation. Then you learn to leave it as is.

(1) just tried it. Even with 'sync' I can't seem to free everything. Linux still "eats" my RAM!!! Any explanation?

At least one cause I know of is applications creating an anonymous mmap() region. IIRC that's counted by the nr_active_anon field in /proc/vmstat

It did not eat my RAM, but ext4 had breakfast with production data.

This made my monday.