On Fri, Feb 12, 2021 at 03:07:39PM -0800, Ian Lance Taylor wrote: > On Fri, Feb 12, 2021 at 3:03 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > > 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 <gregkh@xxxxxxxxxxxxxxxxxxx> 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. > > OK, we'll take it out. > > I'll just make one last plea that I think that copy_file_range could > be much more useful if there were some way that a program could know > whether it would work or not. If you can't tell from userspace that a file has data in it other than by calling read() on it, then you can't use cfr on it. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
