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Subject: Re: [PATCH] mm: remove extra ZONE_DEVICE struct page refcount
To:     Christoph Hellwig <hch@lst.de>
CC:     Christoph Hellwig <hch@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
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From:   Ralph Campbell <rcampbell@nvidia.com>
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On 9/15/20 10:36 PM, Christoph Hellwig wrote:
> On Tue, Sep 15, 2020 at 09:39:47AM -0700, Ralph Campbell wrote:
>>> I don't think any of the three ->page_free instances even cares about
>>> the page refcount.
>>>
>> Not true. The page_free() callback records the page is free by setting
>> a bit or putting the page on a free list but when it allocates a free
>> device private struct page to be used with migrate_vma_setup(), it needs to
>> increment the refcount.
>>
>> For the ZONE_DEVICE MEMORY_DEVICE_GENERIC and MEMORY_DEVICE_PCI_P2PDMA
>> struct pages, I think you are correct because they don't define page_free()
>> and from what I can see, don't decrement the page refcount to zero.
> 
> Umm, the whole point of ZONE_DEVICE is to have a struct page for
> something that is not system memory.  For both the ppc kvm case (magic
> hypervisor pool) and Noveau (device internal) memory that clear is the
> case.  But looks like test_hmm uses normal pages to fake this up, so
> I was wrong about the third caller.  But I think we can just call
> set_page_count just before freeing the page there with a comment
> explaining what is goin on.

Dan Williams thought that having the ZONE_DEVICE struct pages
be on a free list with a refcount of one was a bit strange and
that the driver should handle the zero to one transition.
But, that would mean a bit more invasive change to the 3 drivers
to set the reference count to zero after calling memremap_pages()
and setting the reference count to one when allocating a struct
page. What you are suggesting is what I also proposed in v1.