[$] How useful should copy_file_range() be?
The copy_file_range()
system call looks like a relatively straightforward feature; it allows
user space to ask the kernel to copy a range of data from one file to
another, hopefully applying some optimizations along the way. In truth,
this call has never been as generic as it seems, though some changes made
during 5.3 helped in that regard. When the developers of the Go language
ran into problems with copy_file_range(), there ensued a lengthy
discussion on how this system call should work and whether the kernel needs
to do more to make it useful.