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

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On Mon, 2018-12-03 at 12:31 -0800, Dan Williams wrote:
> On Mon, Dec 3, 2018 at 12:21 PM Alexander Duyck
> <alexander.h.duyck@linux.intel.com> wrote:
> > 
> > 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?
> 
> I don't understand how HMM expects to enforce no pinning, but in any
> event it should always be the expectation an elevated reference count
> on a page prevents that page from disappearing. Anything else is
> broken.

I don't think that is true for device MMIO though.

In the case of MMIO you have the memory region backed by a device, if
that device is hot-plugged or fails in some way then that backing would
go away and the reads would return and all 1's response.

Holding a reference to the page doesn't guarantee that the backing
device cannot go away. I believe that is the origin of the original use
of the PageReserved check in KVM in terms of if it will try to use the
get_page/put_page functions. I believe this is also why
MEMORY_DEVICE_PUBLIC specifically calls out that you should not allow
pinning such memory.

- Alex

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