Re: [PATCH v1 02/11] VFS permit cross device vfs_copy_file_range

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2018-10-19 at 12:06 -0700, Matthew Wilcox wrote:
> On Fri, Oct 19, 2018 at 02:47:42PM -0400, Olga Kornievskaia wrote:
> > On Fri, Oct 19, 2018 at 1:58 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > 
> > > On Fri, Oct 19, 2018 at 11:30:18AM -0400, Olga Kornievskaia wrote:
> > > > +++ b/Documentation/filesystems/vfs.txt
> > > > @@ -958,7 +958,9 @@ otherwise noted.
> > > > 
> > > >    fallocate: called by the VFS to preallocate blocks or punch a hole.
> > > > 
> > > > -  copy_file_range: called by the copy_file_range(2) system call.
> > > > +  copy_file_range: called by copy_file_range(2) system call. This method
> > > > +                works on two file descriptors that might reside on
> > > > +                different superblocks of the same type of file system.
> > > 
> > > I don't think this text is explicit enough about what has changed,
> > 
> > Can you suggest a different wording that would be stronger? I can't
> > say any more clear that copy is allowed between different superblock
> > which wasn't the case before.
> 
> I would leave this file alone.
> 
> > > and I
> > > think this is the wrong place for it.  I think there should be a paragraph
> > > in Documentation/filesystems/porting and it should follow the current style
> > > in there.
> > 
> > I have looked at the "porting" file and it's cryptic. I don't
> > understand what [mandatory] [recommended] stanzas are. There is
> > currently no copy_file_range. Is this just a "changelog" and you are
> > looking for something like
> > [mandatory]
> > copy_file_range() now allows copying between different superblocks.
> > 
> > I can add this wording to the "porting" file.
> 
> The porting file is written from the point of view of someone who's trying
> to port an old filesystem to current Linux.
> 
> Maybe something like
> 
> [mandatory]
> 	->copy_file_range() may now be passed files which belong to two
> 	different filesystems.  The destination's copy_file_range() is the
> 	function which is called.  If it cannot copy ranges from the source,
> 	it should return -EXDEV.
> 
> > > > -     if (file_out->f_op->copy_file_range) {
> > > > +     if (file_out->f_op->copy_file_range &&
> > > > +                     (file_in->f_op->copy_file_range ==
> > > > +                             file_out->f_op->copy_file_range)) {
> > > 
> > > Can we avoid this extra test here?  I know the various stamdards groups
> > > including T10 and the IETF have been trying to define a universal
> > > identifier for the same blob of storage, no matter how it's accessed;
> > > potentially allowing access to the same storage across iSCSI, CIFS
> > > and NFS.  If we ever get to a point where we support that (and I am
> > > dubious), we'd want to remove this test again, and have to revalidate
> > > all the filesystems.
> > 
> > Well from previous submissions I was explicitly asked to add this check.
> 
> I'm not sure that's exactly what Jeff was asking for.  Or maybe it was
> and my argument above will change his mind.  Jeff, what do you think?
> 
> If we do do this, cifs will need a modification as part of this patch to
> reject non-CIFS files as it currently assumes that src_file->private_data
> is a pointer to a struct cifsFileInfo.

My suggestion to Olga was more of a "if you feel you have to have a
check like this at the vfs layer...", but I think you and Al have
convinced me that comparing operations like this is not a good idea.

I still think that this check belongs down inside the fs copy_file_range
operation itself. That allows the the freedom to implement xdev copies
on a per-fs basis later without needing to alter vfs-level code.
-- 
Jeff Layton <jlayton@kernel.org>