There is a database that uses `mmap()` - RavenDB. Their memory accounting is utter horror - they somehow use Commited_AS from /proc/meminfo in their calculations. Their recommendation to avoid OOMs is to have swap twice the size of RAM. Their Jepsen test results are pure comedy.
LMDB uses mmap() as well, but it only supports one process holding the database open at a time. It's also not intended for working sets larger than available RAM.