Subject: Re: [PATCH v1 0/8] VFS: In-kernel copy system call
To: Chris Mason <clm@fb.com>,
        Trond Myklebust <trond.myklebust@primarydata.com>,
        Andy Lutomirski <luto@amacapital.net>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        =?UTF-8?Q?P=c3=a1draig_Brady?= <P@draigbrady.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        Linux btrfs Developers List <linux-btrfs@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>, Zach Brown <zab@zabbo.net>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
        William Andros Adamson <andros@netapp.com>,
        Christoph Hellwig <hch@infradead.org>, Coreutils <coreutils@gnu.org>
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From: Anna Schumaker <Anna.Schumaker@netapp.com>
Message-ID: <55F0997E.1040105@Netapp.com>
Date: Wed, 9 Sep 2015 16:41:34 -0400
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On 09/09/2015 04:38 PM, Chris Mason wrote:
> On Wed, Sep 09, 2015 at 04:26:58PM -0400, Trond Myklebust wrote:
>> On Wed, Sep 9, 2015 at 4:09 PM, Chris Mason <clm@fb.com> wrote:
>>> On Tue, Sep 08, 2015 at 04:08:43PM -0700, Andy Lutomirski wrote:
>>>> On Tue, Sep 8, 2015 at 3:39 PM, Darrick J. Wong <darrick.wong@oracle.com> wrote:
>>>>> On Tue, Sep 08, 2015 at 02:45:39PM -0700, Andy Lutomirski wrote:
>>>>>> What I meant by this was: if you ask for "regular copy", you may end
>>>>>> up with a reflink anyway.  Anyway, how can you reflink a range and
>>>>>> have the contents *not* be the same?
>>>>>
>>>>> reflink forcibly remaps fd_dest's range to fd_src's range.  If they didn't
>>>>> match before, they will afterwards.
>>>>>
>>>>> dedupe remaps fd_dest's range to fd_src's range only if they match, of course.
>>>>>
>>>>> Perhaps I should have said "...if the contents are the same before the call"?
>>>>>
>>>>
>>>> Oh, I see.
>>>>
>>>> Can we have a clean way to figure out whether two file ranges are the
>>>> same in a way that allows false negatives?  I.e. return 1 if the
>>>> ranges are reflinks of each other and 0 if not?  Pretty please?  I've
>>>> implemented that in the past on btrfs by syncing the ranges and then
>>>> comparing FIEMAP output, but that's hideous.
>>>
>>> I'd almost rather have a separate call, maybe unshare_file_range()?
>>>
>>
>> Doesn't it make more sense to put that functionality in fallocate()?
> 
> That works too, I'm just hoping to keep the copy_file_range stuff
> simple.

I agree with keeping copy_file_range() simple, especially for the initial merge.  Extra stuff can always be added in later :)

Anna

> 
> -chris
>