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.
