Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page() with a single reference

From	Dan Williams <dan.j.williams@intel.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page() with a single reference
Date	2017-05-01 03:50 +0200
Message-ID	<tC8Dn-2ss-5@gated-at.bofh.it> (permalink)
References	(6 earlier) <tBjAR-3oJ-5@gated-at.bofh.it> <tBjKy-3t4-19@gated-at.bofh.it> <tBjUe-3ze-21@gated-at.bofh.it> <tBxDQ-4Zx-11@gated-at.bofh.it> <tC6ie-15I-3@gated-at.bofh.it>
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On Sun, Apr 30, 2017 at 4:14 PM, Jerome Glisse <jglisse@redhat.com> wrote:
> On Sat, Apr 29, 2017 at 01:17:26PM +0300, Kirill A. Shutemov wrote:
>> On Fri, Apr 28, 2017 at 03:33:07PM -0400, Jerome Glisse wrote:
>> > On Fri, Apr 28, 2017 at 12:22:24PM -0700, Dan Williams wrote:
>> > > Are you sure about needing to hook the 2 -> 1 transition? Could we
>> > > change ZONE_DEVICE pages to not have an elevated reference count when
>> > > they are created so you can keep the HMM references out of the mm hot
>> > > path?
>> >
>> > 100% sure on that :) I need to callback into driver for 2->1 transition
>> > no way around that. If we change ZONE_DEVICE to not have an elevated
>> > reference count that you need to make a lot more change to mm so that
>> > ZONE_DEVICE is never use as fallback for memory allocation. Also need
>> > to make change to be sure that ZONE_DEVICE page never endup in one of
>> > the path that try to put them back on lru. There is a lot of place that
>> > would need to be updated and it would be highly intrusive and add a
>> > lot of special cases to other hot code path.
>>
>> Could you explain more on where the requirement comes from or point me to
>> where I can read about this.
>>
>
> HMM ZONE_DEVICE pages are use like other pages (anonymous or file back page)
> in _any_ vma. So i need to know when a page is freed ie either as result of
> unmap, exit or migration or anything that would free the memory. For zone
> device a page is free once its refcount reach 1 so i need to catch refcount
> transition from 2->1
>
> This is the only way i can inform the device that the page is now free. See
>
> https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-v21&id=52da8fe1a088b87b5321319add79e43b8372ed7d
>
> There is _no_ way around that.

Ok, but I need to point out that this not a ZONE_DEVICE requirement.
This is an HMM-specific need. So, this extra reference counting should
be clearly delineated as part of the MEMORY_DEVICE_PRIVATE use case.

Can we hide the extra reference counting behind a static branch so
that the common case fast path doesn't get slower until a HMM device
shows up?

Thread

[PATCH] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-04-27 03:10 +0200
  Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-04-27 10:40 +0200
    Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Ingo Molnar <mingo@kernel.org> - 2017-04-28 08:40 +0200
      [PATCH] mm, zone_device: Replace {get, put}_zone_device_page() with a single reference "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2017-04-28 10:20 +0200
      [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-04-28 19:30 +0200
        Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-04-28 19:40 +0200
          Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-04-28 19:50 +0200
            Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-04-28 20:10 +0200
              Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-04-28 21:10 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-04-28 21:20 +0200
                Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-04-28 21:30 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-04-28 21:40 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-04-29 12:20 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-05-01 01:20 +0200
                Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-05-01 03:50 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-05-01 04:00 +0200
                Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-05-01 04:50 +0200
                Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Logan Gunthorpe <logang@deltatee.com> - 2017-05-01 05:50 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-05-01 12:50 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-05-01 16:00 +0200
                Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-05-01 22:30 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-05-01 22:40 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-05-02 13:50 +0200
                Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Jerome Glisse <jglisse@redhat.com> - 2017-05-02 15:30 +0200
        Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Ingo Molnar <mingo@kernel.org> - 2017-04-29 16:20 +0200
          Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-05-01 04:50 +0200
            Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference Ingo Molnar <mingo@kernel.org> - 2017-05-01 09:20 +0200
              Re: [PATCH v2] mm, zone_device: replace {get,  put}_zone_device_page() with a single reference "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-05-01 11:40 +0200
        [tip:x86/mm] mm, zone_device: Replace {get, put}_zone_device_page()  with a single reference to fix pmem crash tip-bot for Dan Williams <tipbot@zytor.com> - 2017-05-01 10:40 +0200
  Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-04-27 18:20 +0200
    Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Logan Gunthorpe <logang@deltatee.com> - 2017-04-27 18:40 +0200
      Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-04-27 18:40 +0200
        Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Dan Williams <dan.j.williams@intel.com> - 2017-04-27 18:50 +0200
        Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Logan Gunthorpe <logang@deltatee.com> - 2017-04-27 18:50 +0200
  Re: [PATCH] mm, zone_device: replace {get, put}_zone_device_page()  with a single reference Logan Gunthorpe <logang@deltatee.com> - 2017-04-27 18:20 +0200

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