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

From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "Jérôme Glisse" <jglisse@redhat.com>,
	"Barret Rhoden" <brho@google.com>,
	"Zhang, Yu C" <yu.c.zhang@intel.com>,
	"KVM list" <kvm@vger.kernel.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	"Jan Kara" <jack@suse.cz>, "David Hildenbrand" <david@redhat.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Linux MM" <linux-mm@kvack.org>,
	rkrcmar@redhat.com, "Paolo Bonzini" <pbonzini@redhat.com>,
	"Christoph Hellwig" <hch@lst.de>
Subject: Re: [PATCH RFC 2/3] mm: Add support for exposing if dev_pagemap supports refcount pinning
Date: Mon, 03 Dec 2018 12:21:42 -0800	[thread overview]
Message-ID: <97943d2ed62e6887f4ba51b985ef4fb5478bc586.camel@linux.intel.com> (raw)
In-Reply-To: <CAPcyv4gZkx9zRsKkVhrmPG7SyjPEycp0neFnECmSADZNLuDOpQ@mail.gmail.com>

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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