On Mon, Jul 17, 2023 at 04:54:58PM +0100, Matthew Wilcox wrote: > Those are page->flags and page->compound_head for the third page in > the folio. They don't really need a name; nothing refers to them, > but it's important that space not be reused ;-) > > This is slightly different from _flags_1; we do have some flags which > reuse the bits (they're labelled as PF_SECOND). Right now, it's only > PG_has_hwpoisoned, but we used to have PG_double_map. Others may arise. Sorry, this was incomplete. We do still have per-page flags! HWPoison is the obvious one, but PG_head is per-page (... think about it ...) PG_anon_exclusive is actually per-page. Most of the flags labelled as PF_ANY are mislabelled. PG_private and PG_private2 are never set/cleared/tested on tail pages. PG_young and PG_idle are only ever tested on the head page, but some code incorrectly sets them on tail pages, where those bits are ignored. I tried to fix that a while ago, but the patch was overlooked and I couldn't be bothered to try all that hard. I have no clue about PG_vmemmap_self_hosted. I think PG_isolated is probably never set on compound pages. PG_owner_priv_1 is a disaster, as you might expect.
