Date: Fri, 12 Oct 2018 09:07:59 -0700
From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Dave Chinner <david@fromorbit.com>
Cc: sandeen@redhat.com, linux-nfs@vger.kernel.org,
        linux-cifs@vger.kernel.org, linux-unionfs@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-mm@kvack.org,
        linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        ocfs2-devel@oss.oracle.com
Subject: Re: [PATCH 05/25] vfs: avoid problematic remapping requests into
 partial EOF block
Message-ID: <20181012160759.GF28243@magnolia>
References: <153923113649.5546.9840926895953408273.stgit@magnolia>
 <153923117420.5546.13317703807467393934.stgit@magnolia>
 <20181012001615.GR6311@dastard>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
In-Reply-To: <20181012001615.GR6311@dastard>
Sender: linux-nfs-owner@vger.kernel.org

On Fri, Oct 12, 2018 at 11:16:16AM +1100, Dave Chinner wrote:
> On Wed, Oct 10, 2018 at 09:12:54PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > A deduplication data corruption is exposed by fstests generic/505 on
> > XFS. It is caused by extending the block match range to include the
> > partial EOF block, but then allowing unknown data beyond EOF to be
> > considered a "match" to data in the destination file because the
> > comparison is only made to the end of the source file. This corrupts the
> > destination file when the source extent is shared with it.
> > 
> > The VFS remapping prep functions only support whole block dedupe, but
> > we still need to appear to support whole file dedupe correctly.  Hence
> > if the dedupe request includes the last block of the souce file, don't
> > include it in the actual dedupe operation. If the rest of the range
> > dedupes successfully, then reject the entire request.  A subsequent
> > patch will enable us to shorten dedupe requests correctly.
> 
> Ok, so this patch rejects whole file dedupe requests, and then a
> later patch adds support back in for it?
> 
> Doesn't that leave a bisect landmine behind? Why separate the
> functionality like this?

Heh, it's a leftover from when I was trying to undo the behavior that
bytes_deduped == len even if we rounded down.  I gave up on that, so
this can match the xfs patch.

--D

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com