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

[Alexander Duyck]

On Mon, 2018-12-03 at 13:05 -0800, Dan Williams wrote:
> On Mon, Dec 3, 2018 at 12:53 PM Alexander Duyck
> <alexander.h.duyck@xxxxxxxxxxxxxxx> wrote:
> > 
> > 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >  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.
> 
> Until p2pdma there are no struct pages for device memory, is that what
> you're referring?

Honestly I am not sure. It is possible I am getting beyond my depth.

My understanding is that we have a 'struct page' for any of these pages
that we are currently using. It is just a matter of if we want to pass
the struct page around or not. So for example in the case of an MMIO
page we still have a 'struct page', however the PG_reserved flag is set
on such a page, so KVM is opting to not touch the reference count,
modify the dirty/accessed bits, and is generally reducing performance
as a result.

> Otherwise any device driver that leaks "struct pages" into random code
> paths in the kernel had better not expect to be able to
> surprise-remove those pages from the system. Any dev_pagemap user
> should expect to do a coordinated removal with the driver that waits
> for page references to drop before the device can be physically
> removed.

Right. This part I get. However I would imagine there still has to be
some exception handling in the case of a PCIe backed region of memory
so that if the device falls of the bus we clean up the dev_pagemap
memory.

> > Holding a reference to the page doesn't guarantee that the backing
> > device cannot go away.
> 
> Correct there is no physical guarantee, but that's not the point. It
> needs to be coordinated, otherwise all bets are off with respect to
> system stability.

Right.

> > 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.
> 
> Is it? MMIO does not typically have a corresponding 'struct page'.

I think we might be talking about different things when we say 'struct
page'. I'm pretty sure there has to be a 'struct page' for the MMIO
region as otherwise we wouldn't be able to check for the PG_reserved
bit in the 'struct page'. Do you maybe mean that MMIO doesn't have a
corresponding virtual address or TLB entry? I know that is what we are
normally generating via the ioremap family of calls in device drivers
in order to access such memory if I am not mistaken.

> > I believe this is also why
> > MEMORY_DEVICE_PUBLIC specifically calls out that you should not allow
> > pinning such memory.
> 
> I don't think that call out was referencing device hotplug, I believe
> it was the HMM expectation that it should be able to move an HMM page
> from device to System-RAM at will.

I could be wrong. If so that would make this patch set easier since
essentially it would just mean that any PageReserved page that matches
is_zone_device_page would fall into this category then and I could just
drop patch 2, and probably combine the entire fix for all of this into
one patch as it would only really be a few additional lines.