Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated

[Dave Chinner]

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