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

["Dan Williams (nvidia)" <djbw@kernel.org>]

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().

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?

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

Feel free to add:

Reviewed-by: Dan Williams <djbw@kernel.org>

...to this and the previous patches when that is fixed up.