It's right in the documentation for mmap() [0]! And, from my experience, using it with an 800GB file provided a significant speed-up, so I do believe the documentation is correct ;)

And, you can poke around in the linux kernel's source code to determine how it works. I had a related issue that I ended up digging around to find the answer to: what happens if you use mremap() to expand the mapping and it fails; is the old mapping still valid or not? Answer: it's still valid. I found that it was actually fairly easy to read linux kernel C code, compared to a lot (!) of other C libraries I've tried to understand.

[0]: https://www.man7.org/linux/man-pages/man2/mmap.2.html