Re: mmap with MAP_PRIVATE is useless

Paolo Bonzini [off-list ref] writes:

This means that MAP_PRIVATE is utterly useless.

I do not think we ever used MAP_PRIVATE in order to protect outselves from
uncontrolled changes made by the outside world in the first place.  Back
when most of these mmap calls were written by Linus and myself, we weren't
interested in using MAP_PRIVATE, or any other trick for that matter, to
deal with the case where the user tells git to go index a file, and then
mucks with the file before git finishes and gives back control.

We do use mmap in read-write mode when reading from the index file, and we
use MAP_PRIVATE to protect the outside world from our writing into the
mapped memory.  As far as I know that is the only mmap for which
MAP_PRIVATE matters in the core git codebase.

Our calls to mmap() almost all have MAP_PRIVATE, even for read-only mmap,
but that is more or less from inertia, aka "an existing call to mmap is
with these options, I'll add another call imitating that".