From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@lst.de>
Date: Thu, 15 Oct 2020 07:44:23 +0000
Subject: Re: [PATCH v2] mm/hmm: make device private reference counts zero based
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To: Dan Williams <dan.j.williams@intel.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>, Linux MM <linux-mm@kvack.org>, kvm-ppc@vger.kernel.org, nouveau@lists.freedesktop.org, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Ira Weiny <ira.weiny@intel.com>, Matthew Wilcox <willy@infradead.org>, Jerome Glisse <jglisse@redhat.com>, John Hubbard <jhubbard@nvidia.com>, Alistair Popple <apopple@nvidia.com>, Christoph Hellwig <hch@lst.de>, Jason Gunthorpe <jgg@nvidia.com>, Zi Yan <ziy@nvidia.com>, "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, Yang Shi <yang.shi@linux.alibaba.com>, Paul Mackerras <paulus@ozlabs.org>, Ben Skeggs <bskeggs@redhat.com>, Andrew Morton <akpm@linux-foundation.org>

On Mon, Oct 12, 2020 at 02:14:07PM -0700, Dan Williams wrote:
> > ZONE_DEVICE struct pages have an extra reference count that complicates the
> > code for put_page() and several places in the kernel that need to check the
> > reference count to see that a page is not being used (gup, compaction,
> > migration, etc.). Clean up the code so the reference count doesn't need to
> > be treated specially for device private pages, leaving DAX as still being
> > a special case.
> 
> Please no half-step to removing the special casing...

Agreed.  I really like where the refcount changes are heading, but
I don't think this half-step is helpful.