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Date:   Thu, 12 Oct 2017 11:37:22 -0400
From:   Jerome Glisse <jglisse@redhat.com>
To:     Bob Liu <lliubbo@gmail.com>
Cc:     Bob Liu <liubo95@huawei.com>,
        Dan Williams <dan.j.williams@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        John Hubbard <jhubbard@nvidia.com>,
        David Nellans <dnellans@nvidia.com>,
        Balbir Singh <bsingharora@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 0/6] Cache coherent device memory (CDM) with HMM v5
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On Wed, Oct 11, 2017 at 09:15:57PM +0800, Bob Liu wrote:
> On Sun, Oct 1, 2017 at 6:49 AM, Jerome Glisse <jglisse@redhat.com> wrote:
> > 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@redhat.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@redhat.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@redhat.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.
> >
> 
> Thank you.

I forgot about this, sorry i was traveling i am still catching up. I will send
you those patches once i unearth where i end up backing them.

> 
> >>
> >> > 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 ?
> >
> 
> You may think it as a CCIX device or CAPI device.
> The requirement is eliminate any extra copy.
> A typical usecase/requirement is malloc() and madvise() allocate from
> device memory, then CPU write data to device memory directly and
> trigger device to read the data/do calculation.

I suggest you rely on the device driver userspace API to do a migration after malloc
then. Something like:
  ptr = malloc(size);
  my_device_migrate(ptr, size);

Which would call an ioctl of the device driver which itself would migrate memory or
allocate device memory for the range if pointer return by malloc is not yet back by
any pages.

There has been several discussions already about madvise/mbind/set_mempolicy/
move_pages and at this time i don't think we want to add or change any of them to
understand device memory. My personal opinion is that we first need to have enough
upstream user and understand of how it is actualy use before it make sense to try
to formalize and define a syscall or change an existing one. User facing API are
set in stone and i don't want to design them by making broad assumption on how i
think device memory will be use.

So for time being i think it is better to use existing device API to manage and
give hint to the kernel on where memory should be (ie should device memory be use
for some range). The first user of this are GPU and they already have a lot of
ioctl to manage and propagate hint from user space. So at this time i suggest that
you piggy back on any existing ioctl of your device or add new ioctl.

Hope this help.
J�r�me

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