Quoting Andrew Cagney,

> Operating systems that implement this include most modern flavours of
> UNIX (the ones I know include SunOS, Solaris, OSF/1, HPUX, Linux
                                                             ^^^^^

It is actually a common misconception that Linux has it. Linux has a buffer
cache that grows and shrinks with the availability of memory, but it is
not a unified cache. What's worse, it did not have msync() for a long
time, so there was no way to sync things manually either. Msync() has only
recently been added as a system call.

- Frank