Re: [PATCH 05/25] vfs: avoid problematic remapping requests into partial EOF block

Darrick J. Wong
2018-10-12T16:07:59Z

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