I was able to spend more time on that this afternoon.
'Got an unkillable diff(1) along the way which required me to
force a cold reboot.  Well.

I attach a C version (11277.mmap.c) which i've used for testing.
The file 11277.zsum32.c is a quick-and-dirty C program to
calculate CRC-32 and Adler-32 checksums (i had none for the
latter and maybe you want to test some more, so); it requires zlib.
I also attach 11277.1.diff which updates test/test_zlib.py, though
this is rather useless, because that still results in a bus error.

This is the real interesting thing however, because the C version
actually works quite well for the chosen value, and the resulting
files are identical, as zsum32 shows:

    Adler-32 <14b9018b> CRC-32 <c6e340bf> -- test_python_413/@test_413_tmp
    Adler-32 <14b9018b> CRC-32 <c6e340bf> -- c-mmap-testfile

I thought
             os.fsync(f.fileno())
does the trick because it does it in C (hi, Charles-Francois),
but no.
So what do i come up with?
Nothing.  A quick look into 11277.mmap.c will show you this:

    /* *Final* sizes (string written after lseek(2): "abcd") */
...
        /* Tested good */
        //0x100000000 - PAGESIZE - 5,
        //0x100000000 - 4,
        //0x100000000 - 3,
        //0x100000000 - 1,
        0x100000000 + PAGESIZE + 4,
        //0x100000000 + PAGESIZE + 5,
        /* Tested bad */
        //0x100000000,
        //0x100000000 + PAGESIZE,
        //0x100000000 + PAGESIZE + 1,
        //0x100000000 + PAGESIZE + 3,

Hm!
Now i have to go but maybe i can do some more testing tomorrow to
answer the question why test_zlib.py fails even though there is
the fsync() and even though the values work in C.
Any comments?