mmap fails to update atime
it seems mmap() fails to update the atime of files correctly, as mandated by SUSv3:
> The st_atime field of the mapped file may be marked for update at any time between the mmap() call and the corresponding munmap() call. The initial read or write reference to a mapped region shall cause the file's st_atime field to be marked for update if it has not already been marked for update.
more details follow.