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

[Kani, Toshimitsu]

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 at hpe.com> 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 at redhat.com> 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. 
 
> > About protecting direct_access, this patch assumes that the underlining
> > device cannot be disabled until dtr() is called.  Is this correct?  If
> > not, I will need to call dax_map_atomic().
>
> One of the big design considerations for DM that a DM device can be
> suspended (with or without flush) and any new IO will be blocked until
> the DM device is resumed.
> 
> So ideally DM should be able to have the same capability even if using
> DAX.

Supporting suspend for DAX is challenging since it allows user applications to
access a device directly.  Once a device range is mmap'd, there is no kernel
intervention to access the range, unless we invalidate user mappings.  This
isn't done today even after a driver is unbind'd from a device.

> But that is different than what commit b2e0d1625e19 is addressing.  For
> DM, I wouldn't think you'd need the extra protections that
> dax_map_atomic() is providing given that the underlying block device
> lifetime is managed via DM core's dm_get_device/dm_put_device (see also:
> dm.c:open_table_device/close_table_device).

I thought so as well.  But I realized that there is (almost) nothing that can
prevent the unbind operation.  It cannot fail, either.  This unbind proceeds
even when a device is in-use.  In case of a pmem device, it is only protected
by pmem_release_queue(), which is called when a pmem device is being deleted
and calls blk_cleanup_queue() to serialize a critical section between
blk_queue_enter() and blk_queue_exit() per b2e0d1625e19.  This prevents from a
kernel DTLB fault, but does not prevent a device disappeared while in-use.

Protecting DM's underlining device with blk_queue_enter() (or something
similar) requires more thoughts...  blk_queue_enter() to a DM device cannot be
redirected to its underlining device.  So, this is TBD for now.  But I do not
think this is a blocker issue since doing unbind to a underlining device is
quite harmful no matter what we do - even if it is protected with
blk_queue_enter().

Thanks,
-Toshi