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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Sun, 12 Apr 2020 14:29:35 +0300
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Subject: Re: Same mountpoint restriction in FICLONE ioctls
To:     Keno Fischer <keno@juliacomputing.com>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        CIFS <linux-cifs@vger.kernel.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        Olga Kornievskaia <kolga@netapp.com>,
        Steve French <smfrench@gmail.com>
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+CC XFS,NFS,CIFS

On Sun, Apr 12, 2020 at 1:06 PM Keno Fischer <keno@juliacomputing.com> wrote:
>
> Hello,
>
> I was curious about the reasoning behind the
> same-mountpoint restriction in the FICLONE
> ioctl. I saw that in commit
>
> [913b86e92] vfs: allow vfs_clone_file_range() across mount points
>
> this check was moved from the vfs layer into
> the ioctl itself, so it appears to be a policy restriction
> rather than a technical limitation. I understand why
> hardlinks are disallowed across mount point boundaries,
> but it seems like that rationale would not apply to clones,
> since modifying the clone would not affect the original
> file. Is there some other reason that the ioctl enforces
> this restriction?
>

I don't know. I suppose that when FICLONE was introduced
there wasn't any use case for cross mount clone.

Note that copy_file_range() also had this restriction, which was
recently lifted, because NFSv4 and CIFS needed this functionality.

As far as I can tell, CIFS and NFSv4 can also support cross mount
clone, but nobody stepped up to request or implement that.

The question is: do you *really* need cross mount clone?
Can you use copy_file_range() instead?
It attempts to do remap_file_range() (clone) before falling back to
kernel copy_file_range().

> Removing this restrictions would have some performance
> advantages for us, but I figured there must be a good reason
> why it's there that I just don't know about, so I figured I'd ask.
>

You did not specify your use case.
Across which filesystems mounts are you trying to clone?

Thanks,
Amir.