linux-ext4 - cross-fs copy support

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Date:   Mon, 1 Oct 2018 20:02:17 +0530
From:   Joshi <joshiiitr@...il.com>
To:     linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: cross-fs copy support

I was wondering about the cross-fs copy through copy_file_range.
It seems current implement has below check, that disables such copy.

1577         /* this could be relaxed once a method supports cross-fs copies */
1578         if (inode_in->i_sb != inode_out->i_sb)
1579                 return -EXDEV;

May I know what are the thoughts behind disabling cross-fs copy?
Code has the comment "once a method supports", but that leaves me
wondering exactly what 'method' is expected, and from whom.

I disabled the check, and copy across volumes seemed to work fine. At
least for a single file (1G size), with no data mismatch, and faster
speed than regular copy.

-- 
Joshi