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SubjectRe: [PATCH RFC 2/3] mm: Add support for exposing if dev_pagemap supports refcount pinning
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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@linux.intel.com> 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@linux.intel.com>
> > ---
> > 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



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