linux-kernel - Re: [PATCH v2] mm/hmm: make device private reference counts zero based

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Date:   Mon, 12 Oct 2020 14:14:07 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     Linux MM <linux-mm@...ck.org>, kvm-ppc@...r.kernel.org,
        nouveau@...ts.freedesktop.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Alistair Popple <apopple@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Jason Gunthorpe <jgg@...dia.com>, Zi Yan <ziy@...dia.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Paul Mackerras <paulus@...abs.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] mm/hmm: make device private reference counts zero based

On Mon, Oct 12, 2020 at 10:46 AM Ralph Campbell <rcampbell@...dia.com> 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...

[..]
> +void free_zone_device_page(struct page *page)
> +{
> +       if (!is_device_private_page(page))
>                 return;

That seems too subtle to be acceptable to me. All ZONE_DEVICE pages
need to have the same relationship with respect to idle-ness and the
page reference count.