What is copy_regular_files? All I found is some Rust code which does not use a single syscall as claimed (it takes file descriptors, so you need to open/close it anyway).

> uses copy acceleration when the file system supports it

I thought that refers to clone_file_range, but that doesn't seem to be the case. As such, I would recommend looking into that, I believe it's supported on btrfs and XFS currently in Linux.

There is also sendfile, but it's probably the same as copy_file_range.