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

[Matthew Wilcox <willy@infradead.org>]

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