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Date: Sat, 29 Apr 2017 13:17:26 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Jerome Glisse <jglisse@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v2] mm, zone_device: replace {get,
 put}_zone_device_page() with a single reference
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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.

-- 
 Kirill A. Shutemov