Date: Wed, 9 Sep 2015 14:42:10 -0700
From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Anna Schumaker <Anna.Schumaker@netapp.com>
Cc: Chris Mason <clm@fb.com>,
        Trond Myklebust <trond.myklebust@primarydata.com>,
        Andy Lutomirski <luto@amacapital.net>,
        =?iso-8859-1?Q?P=E1draig?= 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>
Subject: Re: [PATCH v1 0/8] VFS: In-kernel copy system call
Message-ID: <20150909214210.GC10399@birch.djwong.org>
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On Wed, Sep 09, 2015 at 04:41:34PM -0400, Anna Schumaker wrote:
> 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()?

[slightly off-topic]

How about FALLOC_FL_UNSHARE_RANGE?  I've been looking for a place to land
an unshare op that isn't chattr +C, and fallocate seems like a better fit
anyway.

--D

> > 
> > 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
> > 
> 
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