Is there a difference between mmap MAP_SHARED and MAP_PRIVATE when PROT_READ is also used?

Question

If I create an mmap(2) of a file with a prot parameter of PROT_READ only and the file backing it is also read-only and does not change, is there any performance difference (or any difference at all) between MAP_SHARED and MAP_PRIVATE ? Will the kernel do something differently between the two?

(The documentation only refers to difference of behaviour in terms of "updates", but as it is PROT_READ there can be no updates. I wonder if there is some other difference?)

Answer 1

Under MAP_PRIVATE, the Linux manpage says that it is unspecified whether changes made to the file after the mmap() call are visible in the mapped region. That is not the case with MAP_SHARED. So if you need the contents of the mapping to be updated together with the contents of the file, you had better use MAP_SHARED. If the underlying file itself is read-only and cannot change then of course none of this is applicable.

If PROT_READ is used, I can see nothing else that should be different between MAP_PRIVATE and MAP_SHARED. In fact, despite the above warning about unspecified behaviour, my guess (which I have not tested) would be that in practice there is no difference at all between the two under PROT_READ.