Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5

[Christoph Hellwig]

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.