On Fri, Feb 12, 2021 at 04:45:41PM +0100, Greg KH wrote: > On Fri, Feb 12, 2021 at 07:33:57AM -0800, Ian Lance Taylor wrote: > > On Fri, Feb 12, 2021 at 12:38 AM Greg KH <gre...@linuxfoundation.org> wrote: > > > > > > Why are people trying to use copy_file_range on simple /proc and /sys > > > files in the first place? They can not seek (well most can not), so > > > that feels like a "oh look, a new syscall, let's use it everywhere!" > > > problem that userspace should not do. > > > > This may have been covered elsewhere, but it's not that people are > > saying "let's use copy_file_range on files in /proc." It's that the > > Go language standard library provides an interface to operating system > > files. When Go code uses the standard library function io.Copy to > > copy the contents of one open file to another open file, then on Linux > > kernels 5.3 and greater the Go standard library will use the > > copy_file_range system call. That seems to be exactly what > > copy_file_range is intended for. Unfortunately it appears that when > > people writing Go code open a file in /proc and use io.Copy the > > contents to another open file, copy_file_range does nothing and > > reports success. There isn't anything on the copy_file_range man page > > explaining this limitation, and there isn't any documented way to know > > that the Go standard library should not use copy_file_range on certain > > files. > > But, is this a bug in the kernel in that the syscall being made is not > working properly, or a bug in that Go decided to do this for all types > of files not knowing that some types of files can not handle this? > > If the kernel has always worked this way, I would say that Go is doing > the wrong thing here. If the kernel used to work properly, and then > changed, then it's a regression on the kernel side. > > So which is it?
Both Al Viro and myself have said "copy file range is not a generic method for copying data between two file descriptors". It is a targetted solution for *regular files only* on filesystems that store persistent data and can accelerate the data copy in some way (e.g. clone, server side offload, hardware offlead, etc). It is not intended as a copy mechanism for copying data from one random file descriptor to another. The use of it as a general file copy mechanism in the Go system library is incorrect and wrong. It is a userspace bug. Userspace has done the wrong thing, userspace needs to be fixed. -Dave. -- Dave Chinner da...@fromorbit.com
