[$] How useful should copy_file_range() be?

The copy_file_range()system call looks like a relatively straightforward feature; it allowsuser space to ask the kernel to copy a range of data from one file toanother, hopefully applying some optimizations along the way. In truth,this call has never been as generic as it seems, though some changes madeduring 5.3 helped in that regard. When the developers of the Go languageran into problems with copy_file_range(), there ensued a lengthydiscussion on how this system call should work and whether the kernel needsto do more to make it useful.