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Subject: Re: [PATCH] mm: remove extra ZONE_DEVICE struct page refcount
To:     Christoph Hellwig <hch@infradead.org>
CC:     Dan Williams <dan.j.williams@intel.com>,
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On 9/15/20 9:29 AM, Christoph Hellwig wrote:
> On Mon, Sep 14, 2020 at 04:53:25PM -0700, Ralph Campbell wrote:
>> Since set_page_refcounted() is defined in mm_interal.h I would have to
>> move the definition to someplace like page_ref.h or have the drivers
>> cal init_page_count() or set_page_count() since get_page() calls
>> VM_BUG_ON_PAGE() if refcount == 0.
>> I'll move set_page_refcounted() since that is what the page allocator
>> uses and seems named for the purpose.
> 
> 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.