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

[Jerome Glisse]

On Mon, May 01, 2017 at 01:19:24PM -0700, Dan Williams wrote:
> On Mon, May 1, 2017 at 6:55 AM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
> > 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).
> 
> Right, adding HMM references to get_page() and put_page() seems less
> intrusive. Given how uncommon HMM hardware is (insert grumble about no
> visible upstream user of this functionality) I think the 'static
> branch' approach helps mitigate the impact for everything else.
> Looking back, I should have used that mechanism for the pmem use case,
> but it's moot now.

I do not need anything in get_page() all i need is something in put_page()
to catch the 2 -> 1 refcount transition to know when a page is freed.

Cheers,
Jérôme