One could perhaps have an LD_PRELOADED fsync (or whatever) that updates the mtime with clock_gettime() to store it in its full nanosecond precision glory but it's probably not worth the performance penalty. That wouldn't address the mmap issue of course...
Apparently, according to https://stackoverflow.com/questions/14392975/timestamp-accur..., the ext4 driver just uses the cached kernel clock value without the counter correction that gives you a ns-precise value.
One could perhaps have an LD_PRELOADED fsync (or whatever) that updates the mtime with clock_gettime() to store it in its full nanosecond precision glory but it's probably not worth the performance penalty. That wouldn't address the mmap issue of course...