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

[Jerome Glisse <jglisse@redhat.com>]

On Mon, May 01, 2017 at 01:23:59PM +0300, Kirill A. Shutemov wrote:
> On Sun, Apr 30, 2017 at 07:14:24PM -0400, Jerome Glisse 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
> 
> What if we would rework zone device to have pages with refcount 0 at
> start?

That is a _lot_ of work from top of my head because it would need changes
to a lot of places and likely more hot code path that simply adding some-
thing to put_page() note that i only need something in put_page() i do not
need anything in the get page path. Is adding a conditional branch for
HMM pages in put_page() that much of a problem ?


> > 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.
> 
> I'm still not convinced that it's impossible.
> 
> Could you describe lifecycle for pages in case of HMM?

Process malloc something, end it over to some function in the program
that use the GPU that function call GPU API (OpenCL, CUDA, ...) that
trigger a migration to device memory.

So in the kernel you get a migration like any existing migration,
original page is unmap, if refcount is all ok (no pin) then a device
page is allocated and thing are migrated to device memory.

What happen after is unknown. Either userspace/kernel driver decide
to migrate back to system memory, either there is an munmap, either
there is a CPU page fault, ... So from that point on the device page
as the exact same life as a regular page.

Above i describe the migrate case, but you can also have new memory
allocation that directly allocate device memory. For instance if the
GPU do a page fault on an address that isn't back by anything then
we can directly allocate a device page. No migration involve in that
case.

HMM pages are like any other pages in most respect. Exception are:
  - no GUP
  - no KSM
  - no lru reclaim
  - no NUMA balancing
  - no regular migration (existing migrate_page)

The fact that minimum refcount for ZONE_DEVICE is 1 already gives
us for free most of the above exception. To convert the refcount to
be like other pages would mean that all of the above would need to
be audited and probably modify to ignore ZONE_DEVICE pages (i am
pretty sure Dan do not want any of the above either).

Cheers,
Jérôme