linux-ext4 - Re: cross-fs copy support

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Date:   Mon, 1 Oct 2018 10:49:19 -0500
From:   Eric Sandeen <sandeen@...deen.net>
To:     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 10/1/18 9:48 AM, Qu Wenruo wrote:
> 
> 
> On 2018/10/1 下午10:32, Joshi wrote:
>> I was wondering about the cross-fs copy through copy_file_range.
> 
> The term "cross-fs" looks pretty confusing.
> 
> If you mean "cross-subvolume", then it should work without problem in btrfs.
> 
> If you mean reflink across two different file systems (not matter the
> same fs type or not).
> Then it's impossible to work.

I believe Joshi is talking about vfs_copy_file_range() not
vfs_clone_file range(), although _copy_ does call _clone_ if it can.

> Reflink (clone_file_range) works by inserting data pointers into the
> filesystem other than really copying the data.
> Thus if the source is outside of the fs, it's really impossible to work,
> as the source pointer/data is completely out of control of the dest fs.

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

-Eric

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