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From: Alistair Popple <apopple@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>
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Subject: Re: [RFC 07/10] mm: Allow compound zone device pages
Date: Fri, 12 Apr 2024 11:38:46 +1000
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Matthew Wilcox <willy@infradead.org> writes:

> On Thu, Apr 11, 2024 at 10:57:28AM +1000, Alistair Popple wrote:
>> Supporting compound zone device pages requires compound_head() to
>> distinguish between head and tail pages whilst still preserving the
>> special struct page fields that are specific to zone device pages.
>> 
>> A tail page is distinguished by having bit zero being set in
>> page->compound_head, with the remaining bits pointing to the head
>> page. For zone device pages page->compound_head is shared with
>> page->pgmap.
>> 
>> The page->pgmap field is common to all pages within a memory section.
>> Therefore pgmap is the same for both head and tail pages and we can
>> use the same scheme to distinguish tail pages. To obtain the pgmap for
>> a tail page a new accessor is introduced to fetch it from
>> compound_head.
>
> Would it make sense at this point to move pgmap and zone_device_data
> from struct page to struct folio?  That will make any forgotten
> places fail to compile instead of getting a bogus value.

Thanks for the suggestion, I think that makes a lot of sense. Will try
it for the next version to see what it looks like.