Re: [PATCH 1/1] man-page: copy_file_range(2) allow for cross-device copies

From: Matthew Wilcox <willy@infradead.org>
To: Olga Kornievskaia <olga.kornievskaia@gmail.com>
Cc: Amir Goldstein <amir73il@gmail.com>,
	trond.myklebust@hammerspace.com,
	Anna Schumaker <anna.schumaker@netapp.com>,
	viro@zeniv.linux.org.uk, Steve French <smfrench@gmail.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	linux-nfs <linux-nfs@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, linux-cifs@vger.kernel.org,
	linux-unionfs@vger.kernel.org, linux-man@vger.kernel.org
Subject: Re: [PATCH 1/1] man-page: copy_file_range(2) allow for cross-device copies
Date: Thu, 25 Oct 2018 11:08:00 -0700	[thread overview]
Message-ID: <20181025180800.GJ25444@bombadil.infradead.org> (raw)
In-Reply-To: <CAN-5tyHOk=h+5GdmrUrnTBhvOEeYkz34YXsaudHBiEhgt1o7gQ@mail.gmail.com>

On Thu, Oct 25, 2018 at 01:47:21PM -0400, Olga Kornievskaia wrote:
> On Thu, Oct 25, 2018 at 1:24 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Thu, Oct 25, 2018 at 07:28:10AM +0300, Amir Goldstein wrote:
> > > On Wed, Oct 24, 2018 at 10:59 PM Olga Kornievskaia
> > > <olga.kornievskaia@gmail.com> wrote:
> > > >
> > > > From: Olga Kornievskaia <kolga@netapp.com>
> > > >
> > > > A proposed VFS change removes the check for the files to reside
> > > > under the same file system. Instead, a file system driver implementation
> > > > is allowed to perform a cross-device copy_file_range() and if
> > > > the file system fails to support it instead fallback to doing
> > > > a do_splice copy. Therefore, EXDEV is no longer a possible error.
> > > >
> > > > Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
> > > > ---
> > > >  man2/copy_file_range.2 | 7 ++-----
> > > >  1 file changed, 2 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/man2/copy_file_range.2 b/man2/copy_file_range.2
> > > > index 20374ab..723b2d0 100644
> > > > --- a/man2/copy_file_range.2
> > > > +++ b/man2/copy_file_range.2
> > > > @@ -39,7 +39,8 @@ The
> > > >  .BR copy_file_range ()
> > > >  system call performs an in-kernel copy between two file descriptors
> > > >  without the additional cost of transferring data from the kernel to user space
> > > > -and then back into the kernel.
> > > > +and then back into the kernel. Since kernel version 4.21(???) passed in
> > > > +file descriptors are not required to be under the same mounted file system.
> > > >  It copies up to
> > > >  .I len
> > > >  bytes of data from file descriptor
> > > > @@ -128,10 +129,6 @@ Out of memory.
> > > >  .B ENOSPC
> > > >  There is not enough space on the target filesystem to complete the copy.
> > > >  .TP
> > > > -.B EXDEV
> > > > -The files referred to by
> > > > -.IR file_in " and " file_out
> > > > -are not on the same mounted filesystem.
> > >
> > > Man page serves users of old kernels as well. You should not delete this
> > > expected error, but you can add "... and kernel does not support cross
> > > filesystem copy".
> >
> > I'd rather see this worded as:
> >
> >  .B EXDEV
> >  The files referred to by
> >  .IR file_in " and " file_out
> > -are not on the same mounted filesystem.
> > +cannot be copied due to being on incompatible filesystems
> 
> Why should this be changed to "incompatible filesystems". Existing
> code doesn't support not only incompatibly filesystem but also cross
> device of compatible file systems. And this error is only left for the
> old kernels. In the new kernel, the error EXDEV will never be
> returned.

If I try to copy_file_range() between an NFS filesystem and a CIFS
filesystem with a new kernel, what error do I get?