std.MmFile when opens file in read only mode never expands it, even if memory requested is bigger than actual file size.
When the allocated memory is less than one system page, it passes because mmap() allocates shared memory by page. If however, requested size is bigger than one page, any attempt to read from addresses above page size results to BusError.
Code to illustrate:

unittest // issue: read-only file is not extended
{
    auto fn=deleteme;
    scope(exit) std.file.remove(fn);

    /// create new very short file
    { File(fn,"w").write("123"); }

    /** Trying to map more than one page
     *  The file is not resized and only one page is allocated
     *  Now, the last half of the slice has not valid mapping
    **/
    auto n=new MmFile(fn, MmFile.Mode.read, 0x1020, null);
    auto k=cast(int[]) n[];
    /// Bus error here
    auto y=k[$-1];
}