pppd/tdb.c does this:

        tdb->map_ptr = (void *)mmap(NULL, tdb->map_size, 
                                    tdb->read_only?PROT_READ:PROT_READ|PROT_WRITE,
                                    MAP_SHARED | MAP_FILE, tdb->fd, 0);

If the mmap() fails (some filesystems don't do shared writable mappings) then
mmap will return -1.

Then it does this:

        if (tdb->map_ptr) {
                memcpy(offset + (char *)tdb->map_ptr, buf, len);
        } else {
            /* .... lseek/write ... */
        }

Then it dies.