linux-ext4 - Re: cross-fs copy support

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Date:   Tue, 2 Oct 2018 08:19:26 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     dsterba@...e.cz, Andreas Dilger <adilger@...ger.ca>,
        Eric Sandeen <sandeen@...deen.net>,
        Qu Wenruo <quwenruo.btrfs@....com>,
        Joshi <joshiiitr@...il.com>, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org
Subject: Re: cross-fs copy support

On Tue, Oct 02, 2018 at 10:15:44AM +0200, David Sterba wrote:
> On Mon, Oct 01, 2018 at 01:51:09PM -0600, Andreas Dilger wrote:
> > > Yes, I would expect there to be problems with his modified kernel
> > > for a filesystem that supports clone_file_range, because
> > > vfs_copy_file_range() will clone if possible, and this should fail across
> > > filesystems.
> > > 
> > > In general, though, I don't know for sure why we don't fall back to
> > > do_splice_direct() across filesystems, although the filesystems that
> > > implement their own ->copy_file_range ops may have their own,
> > > further restrictions within their implementations.
> > > 
> > > This call /is/ documented in the manpage as only being valid for
> > > files on the same filesystem, though:
> > > http://man7.org/linux/man-pages/man2/copy_file_range.2.html
> > 
> > There was a patch to allow cross-mount copy for NFS, but it hasn't landed
> > yet.
> 
> I found https://marc.info/?l=linux-nfs&m=144138779721907&w=2 that lifts
> the VFS check (part of a series that can't be easily linked to).
> 
> The lack of cross-mount reflink (based on the copy_file_ragne) is often
> confusing users, there are common setups that mount subvolumes
> separately and reflinking between them would require mount of the
> toplevel subvolume.
> 
> If there are 2 in-kernel users of the relaxed cross-mount copy, I think
> this would help to push the series forward.

I don't have any objection to cross-mountpoint same-filesystem clones,
though obviously we need to all agree that from now on the vfs /does/
support certain IO operations across mountpoints.

(I haven't any opinion on cross-filesystem copies, as XFS is incapable
of such things.)

--D