When I worked on/with BerkeleyDB in the late 90s we came to the conclusion that the various OS mmap() implementations had been tweaked/fixed to the point where they worked for the popular high profile applications (in those days: Oracle). So it can appear like everything is fine, but that probably means your code behaves the same way as <popular database du jour>.
Um... Oracle (and other enterprise databases like DB2) don't use mmap. They use Direct I/O. Oracle does have anonymous (non-file-backed) memory which is mmap'ed and shared across various Oracle processes, called the Shared Global Area (SGA), but it's not used for I/O.
