Please do not use mmap() for reading a file.

According to Linus, with mmap() the actual reading is fast, but settting up
the mmap() is slow, because it invalidates the cache used by the processor
for the translation of virtual addresses to physical ones. So mmap is a
good thing if you call it once in the program, and the read many, many
times. It is also good for sharing the memory overhead of reading a file
between several processes. However, if one is creating and destroying mmaps
all over the time, that is quite slow.