linux-kernel - Re: [PATCH 0/6] Cache coherent device memory (CDM) with HMM v5

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Message-ID: <CAA_GA1ff4mGKfxxRpjYCRjXOvbUuksM0K2gmH1VrhL4qtGWFbw@mail.gmail.com>
Date:   Tue, 26 Sep 2017 17:56:26 +0800
From:   Bob Liu <lliubbo@...il.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Bob Liu <liubo95@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Balbir Singh <bsingharora@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/6] Cache coherent device memory (CDM) with HMM v5

On Tue, Sep 12, 2017 at 7:36 AM, Jerome Glisse <jglisse@...hat.com> wrote:
> On Sun, Sep 10, 2017 at 07:22:58AM +0800, Bob Liu wrote:
>> On Wed, Sep 6, 2017 at 3:36 AM, Jerome Glisse <jglisse@...hat.com> wrote:
>> > On Thu, Jul 20, 2017 at 08:48:20PM -0700, Dan Williams wrote:
>> >> On Thu, Jul 20, 2017 at 6:41 PM, Jerome Glisse <jglisse@...hat.com> wrote:
>> >> > On Fri, Jul 21, 2017 at 09:15:29AM +0800, Bob Liu wrote:
>> >> >> On 2017/7/20 23:03, Jerome Glisse wrote:
>> >> >> > On Wed, Jul 19, 2017 at 05:09:04PM +0800, Bob Liu wrote:
>> >> >> >> On 2017/7/19 10:25, Jerome Glisse wrote:
>> >> >> >>> On Wed, Jul 19, 2017 at 09:46:10AM +0800, Bob Liu wrote:
>> >> >> >>>> On 2017/7/18 23:38, Jerome Glisse wrote:
>> >> >> >>>>> On Tue, Jul 18, 2017 at 11:26:51AM +0800, Bob Liu wrote:
>> >> >> >>>>>> On 2017/7/14 5:15, Jérôme Glisse wrote:
>> >
>> > [...]
>> >
>> >> >> > Second device driver are not integrated that closely within mm and the
>> >> >> > scheduler kernel code to allow to efficiently plug in device access
>> >> >> > notification to page (ie to update struct page so that numa worker
>> >> >> > thread can migrate memory base on accurate informations).
>> >> >> >
>> >> >> > Third it can be hard to decide who win between CPU and device access
>> >> >> > when it comes to updating thing like last CPU id.
>> >> >> >
>> >> >> > Fourth there is no such thing like device id ie equivalent of CPU id.
>> >> >> > If we were to add something the CPU id field in flags of struct page
>> >> >> > would not be big enough so this can have repercusion on struct page
>> >> >> > size. This is not an easy sell.
>> >> >> >
>> >> >> > They are other issues i can't think of right now. I think for now it
>> >> >>
>> >> >> My opinion is most of the issues are the same no matter use CDM or HMM-CDM.
>> >> >> I just care about a more complete solution no matter CDM,HMM-CDM or other ways.
>> >> >> HMM or HMM-CDM depends on device driver, but haven't see a public/full driver to
>> >> >> demonstrate the whole solution works fine.
>> >> >
>> >> > I am working with NVidia close source driver team to make sure that it works
>> >> > well for them. I am also working on nouveau open source driver for same NVidia
>> >> > hardware thought it will be of less use as what is missing there is a solid
>> >> > open source userspace to leverage this. Nonetheless open source driver are in
>> >> > the work.
>> >>
>> >> Can you point to the nouveau patches? I still find these HMM patches
>> >> un-reviewable without an upstream consumer.
>> >
>> > So i pushed a branch with WIP for nouveau to use HMM:
>> >
>> > https://cgit.freedesktop.org/~glisse/linux/log/?h=hmm-nouveau
>> >
>>
>> Nice to see that.
>> Btw, do you have any plan for a CDM-HMM driver? CPU can write to
>> Device memory directly without extra copy.
>
> Yes nouveau CDM support on PPC (which is the only CDM platform commercialy
> available today) is on the TODO list. Note that the driver changes for CDM
> are minimal (probably less than 100 lines of code). From the driver point
> of view this is memory and it doesn't matter if it is CDM or not.
>

It seems have to migrate/copy memory between system-memory and
device-memory even in HMM-CDM solution.
Because device-memory is not added into buddy system, the page fault
for normal malloc() always allocate memory from system-memory!!
If the device then access the same virtual address, the data is copied
to device-memory.

Correct me if I misunderstand something.
@Balbir, how do you plan to make zero-copy work if using HMM-CDM?

--
Thanks,
Bob