linux-kernel - Re: [PATCH 1/6] fs: Add flag to file_system

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Message-ID: <20210212230346.GU4626@dread.disaster.area>
Date:   Sat, 13 Feb 2021 10:03:46 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Ian Lance Taylor <iant@...ang.org>,
        Nicolas Boichat <drinkcat@...omium.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Luis Lozano <llozano@...omium.org>,
        linux-fsdevel@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content
 is generated

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@...uxfoundation.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
david@...morbit.com