Re: [PATCH v6 07/10] dax: plumb hotplug online_type through dax

On Thu, Jul 09, 2026 at 02:46:39PM -0700, Dan Williams (nvidia) wrote:
> Gregory Price wrote:
> > There is no way for drivers leveraging dax_kmem to plumb through a
> > preferred auto-online policy - the system default policy is forced.
> > 
> > Add 'enum mmop' field to DAX device creation path to allow drivers
> > to specify an auto-online policy when using the kmem driver.
> > 
> > Capturing the system default would otherwise break the ABI, because
> > the system default can change - but we would be statically assigning
> > the value at device creation time.
> > 
> > To resolve this we add DAX_ONLINE_DEFAULT, which defaults devices to
> > the current behavior, while providing a clean way to override it.
> > 
> > No behavioural change for existing callers (still the system default).
> 
> So I know you have some future usage for this ability, but it is not
> present in this set. The only piece that *is* used is that the
> online-type from the new sysfs interface gets plumbed through to
> __add_memory_driver_managed().
> 

Correct.

I didn't want to cross three subsystems in one go, I do intend to follow
this up with at least a CXL build option to override the global hotplug
policy by plumbing it through to the existing cxl auto-probe process.

Some of the accelerator stuff is still a bit up in the air but the base
driver can still benefit from this as well.

> Are these touches:
> 
> >  drivers/dax/cxl.c         |  1 +
> >  drivers/dax/hmem/hmem.c   |  1 +
> >  drivers/dax/pmem.c        |  1 +
> 
> ...premature until the first user arrives that with the background story
> about how it knows to set the policy?
>

This was more a matter of having the DEFAULT set consistently across
the dax driver variant probe() functions to make the behavior explicit.
I didn't want an un-set value bug to creep in here somehow.

Happy to drop them if you think that's unneeded.

> If DAX_ONLINE_DEFAULT is a sentinel for "default" should
> DAX_KMEM_UNPLUGGED be a different sentinel than (-1)?
> 

They don't actually run into each other.  DAX_ONLINE_DEFAULT is
overwritten at probe time with the system default policy, so
`dax/state` can never perceive it (even if the values are the same).

But this is visually confusing i suppose, so I'll just swap it for -2.

> Feel free to add:
> 
> Reviewed-by: Dan Williams <[email protected]>
> 
> ...to this and the previous patches when that is fixed up.

Thank you!