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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Tue, 8 Feb 2022 19:30:11 -0800
Message-ID: <CAPcyv4h_axDTmkZ35KFfCdzMoOp8V3dc6btYGq6gCj1OmLXM=g@mail.gmail.com>
Subject: Re: [PATCH 7/8] mm: remove the extra ZONE_DEVICE struct page refcount
To:     Christoph Hellwig <hch@lst.de>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        Ben Skeggs <bskeggs@redhat.com>,
        Karol Herbst <kherbst@redhat.com>,
        Lyude Paul <lyude@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Alistair Popple <apopple@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@lists.freedesktop.org>, nouveau@lists.freedesktop.org,
        Linux NVDIMM <nvdimm@lists.linux.dev>,
        Linux MM <linux-mm@kvack.org>
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On Sun, Feb 6, 2022 at 10:33 PM Christoph Hellwig <hch@lst.de> wrote:
[..]
> @@ -500,28 +482,27 @@ void free_devmap_managed_page(struct page *page)
>          */
>         page->mapping = NULL;
>         page->pgmap->ops->page_free(page);
> +
> +       /*
> +        * Reset the page count to 1 to prepare for handing out the page again.
> +        */
> +       set_page_count(page, 1);

Interesting. I had expected that to really fix the refcount problem
that fs/dax.c would need to start taking real page references as pages
were added to a mapping, just like page cache.

This looks ok to me, and passes my tests. So given I'm still working
my way back to fixing the references properly I'm ok for this hack to
replace the more broken hack that is there presently.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>