Re: [PATCH RFC 2/3] mm: Add support for exposing if dev_pagemap supports refcount pinning

[Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Mon, 2018-12-03 at 11:47 -0800, Dan Williams wrote:
> On Mon, Dec 3, 2018 at 11:25 AM Alexander Duyck
> <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org> wrote:
> > 
> > Add a means of exposing if a pagemap supports refcount pinning. I am doing
> > this to expose if a given pagemap has backing struct pages that will allow
> > for the reference count of the page to be incremented to lock the page
> > into place.
> > 
> > The KVM code already has several spots where it was trying to use a
> > pfn_valid check combined with a PageReserved check to determien if it could
> > take a reference on the page. I am adding this check so in the case of the
> > page having the reserved flag checked we can check the pagemap for the page
> > to determine if we might fall into the special DAX case.
> > 
> > Signed-off-by: Alexander Duyck <alexander.h.duyck-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> > ---
> >  drivers/nvdimm/pfn_devs.c |    2 ++
> >  include/linux/memremap.h  |    5 ++++-
> >  include/linux/mm.h        |   11 +++++++++++
> >  3 files changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c
> > index 6f22272e8d80..7a4a85bcf7f4 100644
> > --- a/drivers/nvdimm/pfn_devs.c
> > +++ b/drivers/nvdimm/pfn_devs.c
> > @@ -640,6 +640,8 @@ static int __nvdimm_setup_pfn(struct nd_pfn *nd_pfn, struct dev_pagemap *pgmap)
> >         } else
> >                 return -ENXIO;
> > 
> > +       pgmap->support_refcount_pinning = true;
> > +
> 
> There should be no dev_pagemap instance instance where this isn't
> true, so I'm missing why this is needed?

I thought in the case of HMM there were instances where you couldn't
pin the page, isn't there? Specifically I am thinking of the definition
of MEMORY_DEVICE_PUBLIC:
  Device memory that is cache coherent from device and CPU point of 
  view. This is use on platform that have an advance system bus (like 
  CAPI or CCIX). A driver can hotplug the device memory using 
  ZONE_DEVICE and with that memory type. Any page of a process can be 
  migrated to such memory. However no one should be allow to pin such 
  memory so that it can always be evicted.

It sounds like MEMORY_DEVICE_PUBLIC and MMIO would want to fall into
the same category here in order to allow a hot-plug event to remove the
device and take the memory with it, or is my understanding on this not
correct?

Thanks.

- Alex