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

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Date:   Sat, 30 Sep 2017 18:49:28 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Bob Liu <liubo95@...wei.com>
Cc:     Bob Liu <lliubbo@...il.com>,
        Dan Williams <dan.j.williams@...el.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 Sat, Sep 30, 2017 at 10:57:38AM +0800, Bob Liu wrote:
> On 2017/9/27 0:16, Jerome Glisse wrote:
> > On Tue, Sep 26, 2017 at 05:56:26PM +0800, Bob Liu wrote:
> >> 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:
> [...]
> >>>>> 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?
> > 
> > Device can access system memory so copy to device is _not_ mandatory. Copying
> > data to device is for performance only ie the device driver take hint from
> > userspace and monitor device activity to decide which memory should be migrated
> > to device memory to maximize performance.
> > 
> > Moreover in some previous version of the HMM patchset we had an helper that
> 
> Could you point in which version? I'd like to have a look.

I will need to dig in.

> 
> > allowed to directly allocate device memory on device page fault. I intend to
> > post this helper again. With that helper you can have zero copy when device
> > is the first to access the memory.
> > 
> > Plan is to get what we have today work properly with the open source driver
> > and make it perform well. Once we get some experience with real workload we
> > might look into allowing CPU page fault to be directed to device memory but
> > at this time i don't think we need this.
> > 
> 
> For us, we need this feature that CPU page fault can be direct to device memory.
> So that don't need to copy data from system memory to device memory.
> Do you have any suggestion on the implementation? I'll try to make a prototype patch.

Why do you need that ? What is the device and what are the requirement ?

Jérôme