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Subject: Re: [RFC PATCH v3 2/2] mm: remove extra ZONE_DEVICE struct page
 refcount
To:     Christoph Hellwig <hch@lst.de>
CC:     <linux-mm@kvack.org>, <kvm-ppc@vger.kernel.org>,
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References: <20201001181715.17416-1-rcampbell@nvidia.com>
 <20201001181715.17416-3-rcampbell@nvidia.com> <20201002055930.GB8891@lst.de>
From:   Ralph Campbell <rcampbell@nvidia.com>
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On 10/1/20 10:59 PM, Christoph Hellwig wrote:
> On Thu, Oct 01, 2020 at 11:17:15AM -0700, Ralph Campbell 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 ZONE_DEVICE.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> 
> Looks good,
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>

Thanks for the review.

I still have reservations about making this an official patch.
Did you see the updated cover letter?
Basically, I'm concerned about ZONE_DEVICE struct pages being inserted into
the process page table with a zero reference count with vmf_insert_mixed().
If it is to be a non-zero reference count, then DAX, pmem, and other uses
of ZONE_DEVICE pages need to be changed (or vmf_insert_mixed()) to
inc/dec in appropriate places but I don't feel I know that code well enough
to make those changes.