How do you do this technically? mremap(), I assume?

You can do something very similar on windows, by VirtualAlloc()ing at the end of the previous allocation, and if that fails, be committing, VirtualFreeing and remapping at a different address.

Yes, I agree mremap() is much cleaner. I am mostly a Linux guy myself. What I'm missing in Linux is "mremap2()", which would map an existing block of memory onto a file (whereas mmap maps an existing file onto a block of memory ). redis achieves similar functionality (and more) by fork()ing and write()ing instead, but it would be nicer if it could be done with one call and no fork()ing. a "copy this file from here to here" could then be done by mmap()ing the source into memory, mremap2()ing the memory into the destination file, and forgetting about it.

added: This would also allow btrfs/zfs to have O(1) file copy, even between independent media if possible (i.e. source file already exists on target media, even if in a different directory) - without any consideration from the cp implementation.