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Re: [HMM v14 05/16] mm/ZONE_DEVICE/unaddressable: add support for un-addressable device memory

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On 12/08/2016 08:39 AM, Jerome Glisse wrote:
>> On 12/08/2016 08:39 AM, Jérôme Glisse wrote:
>>> > > Architecture that wish to support un-addressable device memory should make
>>> > > sure to never populate the kernel linar mapping for the physical range.
>> > 
>> > Does the platform somehow provide a range of physical addresses for this
>> > unaddressable area?  How do we know no memory will be hot-added in a
>> > range we're using for unaddressable device memory, for instance?
> That's what one of the big issue. No platform does not reserve any range so
> there is a possibility that some memory get hotpluged and assign this range.
> 
> I pushed the range decision to higher level (ie it is the device driver that
> pick one) so right now for device driver using HMM (NVidia close driver as
> we don't have nouveau ready for that yet) it goes from the highest physical
> address and scan down until finding an empty range big enough.

I don't think you should be stealing physical address space for things
that don't and can't have physical addresses.  Delegating this to
individual device drivers and hoping that they all get it right seems
like a recipe for disaster.

Maybe worth adding to the changelog:

	This feature potentially breaks memory hotplug unless every
	driver using it magically predicts the future addresses of
	where memory will be hotplugged.

BTW, how many more of these "big issues" does this set have?  I didn't
see any mention of this in the changelogs.

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[HMM v14 00/16] HMM (Heterogeneous Memory Management) v14 Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:40 +0100
  [HMM v14 12/16] mm/hmm/migrate: add new boolean copy flag to migratepage() callback Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:40 +0100
  [HMM v14 01/16] mm/free_hot_cold_page: catch ZONE_DEVICE pages Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 06/16] mm/ZONE_DEVICE/x86: add support for un-addressable device memory Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 03/16] mm/ZONE_DEVICE/devmem_pages_remove: allow early removal of device memory Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 13/16] mm/hmm/migrate: new memory migration helper for use with device memory v2 Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 07/16] mm/hmm: heterogeneous memory management (HMM for short) Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 05/16] mm/ZONE_DEVICE/unaddressable: add support for un-addressable device memory Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
    Re: [HMM v14 05/16] mm/ZONE_DEVICE/unaddressable: add support for  un-addressable device memory Dave Hansen <dave.hansen@intel.com> - 2016-12-08 17:30 +0100
      Re: [HMM v14 05/16] mm/ZONE_DEVICE/unaddressable: add support for  un-addressable device memory Jerome Glisse <jglisse@redhat.com> - 2016-12-08 17:50 +0100
        Re: [HMM v14 05/16] mm/ZONE_DEVICE/unaddressable: add support for  un-addressable device memory Dave Hansen <dave.hansen@intel.com> - 2016-12-08 21:10 +0100
          Re: [HMM v14 05/16] mm/ZONE_DEVICE/unaddressable: add support for  un-addressable device memory Jerome Glisse <jglisse@redhat.com> - 2016-12-08 21:40 +0100
  [HMM v14 02/16] mm/memory/hotplug: convert device bool to int to allow for more flags v2 Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 16/16] mm/hmm/devmem: dummy HMM device as an helper for ZONE_DEVICE memory Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 04/16] mm/ZONE_DEVICE/free-page: callback when page is freed Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 15/16] mm/hmm/devmem: device driver helper to hotplug ZONE_DEVICE memory Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 10/16] mm/hmm/mirror: device page fault handler Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 08/16] mm/hmm/mirror: mirror process address space on device with HMM helpers Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 14/16] mm/hmm/migrate: optimize page map once in vma being migrated Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100
  [HMM v14 09/16] mm/hmm/mirror: helper to snapshot CPU page table Jérôme Glisse <jglisse@redhat.com> - 2016-12-08 16:50 +0100

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