Let me rant for a bit because I've dealt with the following: Another downside is you'll get people complaining about memory usage despite mmap only paging in what's needed.

It's frustrating but users don't understand how to read memory usage at all. There's even guides out there quite wrongly telling users to look at virtual memory usage for each app.

Mmap is a brilliant way to have the os handle which parts of the file actually reside in memory but you'll seriously have to deal with users that know only enough to be dangerous complaining that your app uses gb of ram when it's merely mapping a file into virtual address space to allow the os to page as it sees fit.

In fact I wouldn't at all be surprised if the memory measurements in this thread were judging virtual allocation rather than physical memory used.

Because then all your edits have to go directly into the file, so you have no real "save" flow unless you make a swap file for every file and then mmap that (which can be an appropriate approach, to a degree). Then all your caching and memory use subject to OS I/O and filesystem concerns, and you can't optimize for specific use. And inserting or deleting the middle of the file means shifting all the bytes after it.

I could think of a reasonable implementation for Linux that would use mmap and fallocate, but it wouldn't "just" be an mmap, as the swap file would still need to represent a rope or a gap buffer or something else of the sort for efficient editing.

Aren't edits of a huge file expected to be comparatively small? If you're editing an mmap'd file, I'd expect the editor to maintain a display combining the original mmap'd file with a list of changes that have been performed so far.