Maybe I'm misunderstanding you but: no, you can allocate terabytes of address space on modern 64-bit Linux on a machine with only 8GB of RAM with overcommit. Try it; you can allocate 2^46 bytes of space (~= 100TB) today, with no problem. There is no limit to the allocation space in an overcommit system; there is only a limit to the actual working set, which is very different.

You can do it without overcommit -- you can just back the mmap with file

I don't think so, but it's difficult to find an actual reference. For sure it does overcommit like crazy. Here's an output from my mac:

% ps aux | sort -k5 -rh | head -1

xxxxxxxx 88273 1.2 0.9 1597482768 316064 ?? S 4:07PM 35:09.71 /Applications/Slack.app/Contents/Frameworks/Slack Helper (Renderer).app/...

Since ps displays vsz column in KiB, 1597482768 corresponds to 1TB+.