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

[Jerome Glisse]

On Sun, Apr 30, 2017 at 06:42:02PM -0700, Dan Williams wrote:
> On Sun, Apr 30, 2017 at 4:14 PM, Jerome Glisse <jglisse@xxxxxxxxxx> 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
> >
> > 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.
> 
> Ok, but I need to point out that this not a ZONE_DEVICE requirement.
> This is an HMM-specific need. So, this extra reference counting should
> be clearly delineated as part of the MEMORY_DEVICE_PRIVATE use case.

And it already is delimited, i think you even gave your review by on
the patch.


> Can we hide the extra reference counting behind a static branch so
> that the common case fast path doesn't get slower until a HMM device
> shows up?

Like i already did With likely()/unlikely() ? Or something else ?

https://cgit.freedesktop.org/~glisse/linux/commit/?h=hmm-v21&id=e84778e9db0672e371eb6599dfcb812512118842

Cheers,
Jérôme