Attached is a patch against guile master (at 3.0.4),
commit 5e1748f75128107e3a0707b66df5adb95d98437e

It is a incomplete, but functional, implementation of a mmap-api, including
1) mmap : low-level mmap, returns a bytevector, not searched for roots
2) mmap/search : like mmap, but not marked w/ GC_exclude_static_roots
3) mmap-file: high-level, easy-to-use mmap (e.g., (mmap-file "foo.dat"))

The above are coded in libguile/filesys.[ch].

Also included is test-suite/tests/mmap-api.test.

Build:
$ ./configure --enable-mmap-api
$ make
$ make check
...
Running mmap-api.test
...

Since implementation of mmap may be not simple, I propose a
git branch (e.g., wip-mmap-api) be created to invite group review,
update, test the update.

Matt