Re: [PATCH 0/6] Support DAX for device-mapper dm-linear devices

[Mike Snitzer]

On Wed, Jun 22 2016 at  4:16P -0400,
Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote:

> On Wed, 2016-06-22 at 12:15 -0700, Dan Williams wrote:
> > On Wed, Jun 22, 2016 at 10:44 AM, Kani, Toshimitsu <toshi.kani@xxxxxxx>
> > wrote:
> > > On Tue, 2016-06-21 at 14:17 -0400, Mike Snitzer wrote:
> > > > 
> > > > On Tue, Jun 21 2016 at 11:44am -0400,
> > > > Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote:
> > > > > 
> > > > > On Tue, 2016-06-21 at 09:41 -0400, Mike Snitzer wrote:
> > > > > > On Mon, Jun 20 2016 at  6:22pm -0400,
> > > > > > Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> > > > > > I'm now wondering if we'd be better off setting a new QUEUE_FLAG_DAX
> > > > > > rather than establish GENHD_FL_DAX on the genhd?
> > > > > > 
> > > > > > It'd be quite a bit easier to allow upper layers (e.g. XFS and ext4)
> > > > > > to check for a queue flag.
> > > > > 
> > > > > I think GENHD_FL_DAX is more appropriate since DAX does not use a
> > > > > request queue, except for protecting the underlining device being
> > > > > disabled while direct_access() is called (b2e0d1625e19).
> > > > 
> > > > The devices in question have a request_queue.  All bio-based device have
> > > > a request_queue.
> > >
> > > DAX-capable devices have two operation modes, bio-based and DAX.  I agree
> > > that bio-based operation is associated with a request queue, and its
> > > capabilities should be set to it.  DAX, on the other hand, is rather
> > > independent from a request queue.
> > > 
> > > > I don't have a big problem with GENHD_FL_DAX.  Just wanted to point out
> > > > that such block device capabilities are generally advertised in terms of
> > > > a QUEUE_FLAG.
> > >
> > > I do not have a strong opinion, but feel a bit odd to associate DAX to a
> > > request queue.
> >
> > Given that we do not support dax to a raw block device [1] it seems a
> > gendisk flag is more misleading than request_queue flag that specifies
> > what requests can be made of the device.
> > 
> > [1]: acc93d30d7d4 Revert "block: enable dax for raw block devices"
> 
> Oh, I see.  I will change to use request_queue flag.

I implemented the block patch for this yesterday but based on your
feedback I stopped there (didn't carry the change through to the DM core
and DM linear -- can easily do so tomorrow though).

Feel free to use this as a starting point and fix/extend and add a
proper header: