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

[Kirill A. Shutemov]

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?

> 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?

-- 
 Kirill A. Shutemov