Re: [RFC 07/10] mm: Allow compound zone device pages

Matthew Wilcox
2024-04-11T14:10:29Z

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.