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Date:   Thu, 12 Mar 2020 08:27:29 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Dave Chinner <david@fromorbit.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Ira Weiny <ira.weiny@intel.com>, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations
 V5
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On Thu, Mar 12, 2020 at 11:49:32AM +1100, Dave Chinner wrote:
> > > IOWs, the dax_associate_page() related functionality probably needs
> > > to be a filesystem callout - part of the aops vector, I think, so
> > > that device dax can still use it. That way XFS can go it's own way,
> > > while ext4 and device dax can continue to use the existing mechanism
> > > mechanisn that is currently implemented....
> > 
> > s/XFS/XFS with rmap/, as most XFS file systems currently don't have
> > that enabled we'll also need to keep the legacy path around.
> 
> Sure, that's trivially easy to handle in the XFS code once the
> callouts are in place.
> 
> But, quite frankly, we can enforce rmap to be enabled 
> enabled because nobody is using a reflink enabled FS w/ DAX right
> now. Everyone will have to mkfs their filesystems anyway to enable
> reflink+dax, so we simply don't allow reflink+dax to be enabled
> unless rmap is also enabled. Simple, easy, trivial.

True, I think rmap will be required for DAX+reflink.  But we still
need the legacy infrastructure for error reporting.