At some cost to portability, IIRC, on Linux you can ask the operating system to keep pages in memory. Other systems will probably have similar functionality.
They can remain paged in, but unlike on FreeBSD, you can't control when dirty pages are flushed to the backing filesystem on Linux. Specifically, the MAP_NOSYNC option doesn't exist on Linux (see https://www.freebsd.org/cgi/man.cgi?sektion=2&query=mmap for a description).
Windows has NtLockVirtualMemory, but (a) it requires special permissions (meaning random apps can't do it without admin privileges), and (b) something about it feels like it's the wrong way to do it, but I can't pin down what it is exactly.