Re: [PATCH 6/6] arm64/mm: Enable ZONE_DEVICE

[Dan Williams]

On Thu, Apr 4, 2019 at 2:47 AM Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> On 04/04/2019 06:04, Dan Williams wrote:
> > On Wed, Apr 3, 2019 at 9:42 PM Anshuman Khandual
> > <anshuman.khandual@xxxxxxx> wrote:
> >>
> >>
> >>
> >> On 04/03/2019 07:28 PM, Robin Murphy wrote:
> >>> [ +Dan, Jerome ]
> >>>
> >>> On 03/04/2019 05:30, Anshuman Khandual wrote:
> >>>> Arch implementation for functions which create or destroy vmemmap mapping
> >>>> (vmemmap_populate, vmemmap_free) can comprehend and allocate from inside
> >>>> device memory range through driver provided vmem_altmap structure which
> >>>> fulfils all requirements to enable ZONE_DEVICE on the platform. Hence just
> >>>
> >>> ZONE_DEVICE is about more than just altmap support, no?
> >>
> >> Hot plugging the memory into a dev->numa_node's ZONE_DEVICE and initializing the
> >> struct pages for it has stand alone and self contained use case. The driver could
> >> just want to manage the memory itself but with struct pages either in the RAM or
> >> in the device memory range through struct vmem_altmap. The driver may not choose
> >> to opt for HMM, FS DAX, P2PDMA (use cases of ZONE_DEVICE) where it may have to
> >> map these pages into any user pagetable which would necessitate support for
> >> pte|pmd|pud_devmap.
> >
> > What's left for ZONE_DEVICE if none of the above cases are used?
> >
> >> Though I am still working towards getting HMM, FS DAX, P2PDMA enabled on arm64,
> >> IMHO ZONE_DEVICE is self contained and can be evaluated in itself.
> >
> > I'm not convinced. What's the specific use case.
>
> The fundamental "roadmap" reason we've been doing this is to enable
> further NVDIMM/pmem development (libpmem/Qemu/etc.) on arm64. The fact
> that ZONE_DEVICE immediately opens the door to the various other stuff
> that the CCIX folks have interest in is a definite bonus, so it would
> certainly be preferable to get arm64 on par with the current state of
> things rather than try to subdivide the scope further.
>
> I started working on this from the ZONE_DEVICE end, but got bogged down
> in trying to replace my copied-from-s390 dummy hot-remove implementation
> with something proper. Anshuman has stepped in to help with hot-remove
> (since we also have cloud folks wanting that for its own sake), so is
> effectively coming at the problem from the opposite direction, and I'll
> be the first to admit that we've not managed the greatest job of meeting
> in the middle and coordinating our upstream story; sorry about that :)
>
> Let me freshen up my devmap patches and post them properly, since that
> discussion doesn't have to happen in the context of hot-remove; they're
> effectively just parallel dependencies for ZONE_DEVICE.

Sounds good. It's also worth noting that Ira's recent patches for
supporting get_user_pages_fast() for "longterm" pins relies on
PTE_DEVMAP to determine when fast-GUP is safe to proceed, or whether
it needs to fall back to slow-GUP. So it really is the case that
"devmap" support is an assumption for ZONE_DEVICE.