On 1/16/26 18:20, Jason Gunthorpe wrote: > On Fri, Jan 16, 2026 at 05:07:09PM +0100, Vlastimil Babka wrote: >> On 1/16/26 12:10, Francois Dugast wrote: >> > From: Matthew Brost <[email protected]> >> > diff --git a/mm/memremap.c b/mm/memremap.c >> > index 63c6ab4fdf08..ac7be07e3361 100644 >> > --- a/mm/memremap.c >> > +++ b/mm/memremap.c >> > @@ -477,10 +477,43 @@ void free_zone_device_folio(struct folio *folio) >> > } >> > } >> > >> > -void zone_device_page_init(struct page *page, unsigned int order) >> > +void zone_device_page_init(struct page *page, struct dev_pagemap *pgmap, >> > + unsigned int order) >> > { >> > + struct page *new_page = page; >> > + unsigned int i; >> > + >> > VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES); >> > >> > + for (i = 0; i < (1UL << order); ++i, ++new_page) { >> > + struct folio *new_folio = (struct folio *)new_page; >> > + >> > + /* >> > + * new_page could have been part of previous higher order folio >> > + * which encodes the order, in page + 1, in the flags bits. We >> > + * blindly clear bits which could have set my order field here, >> > + * including page head. >> > + */ >> > + new_page->flags.f &= ~0xffUL; /* Clear possible order, page >> > head */ >> > + >> > +#ifdef NR_PAGES_IN_LARGE_FOLIO >> > + /* >> > + * This pointer math looks odd, but new_page could have been >> > + * part of a previous higher order folio, which sets _nr_pages >> > + * in page + 1 (new_page). Therefore, we use pointer casting to >> > + * correctly locate the _nr_pages bits within new_page which >> > + * could have modified by previous higher order folio. >> > + */ >> > + ((struct folio *)(new_page - 1))->_nr_pages = 0; >> > +#endif >> > + >> > + new_folio->mapping = NULL; >> > + new_folio->pgmap = pgmap; /* Also clear compound head */ >> > + new_folio->share = 0; /* fsdax only, unused for device >> > private */ >> > + VM_WARN_ON_FOLIO(folio_ref_count(new_folio), new_folio); >> > + VM_WARN_ON_FOLIO(!folio_is_zone_device(new_folio), new_folio); >> > + } >> > + >> > /* >> > * Drivers shouldn't be allocating pages after calling >> > * memunmap_pages(). >> >> Can't say I'm a fan of this. It probably works now (so I'm not nacking) but >> seems rather fragile. It seems likely to me somebody will try to change some >> implementation detail in the page allocator and not notice it breaks this, >> for example. I hope we can eventually get to something more robust. > > These pages shouldn't be in the buddy allocator at all? The driver > using the ZONE_DEVICE pages is responsible to provide its own > allocator. > > Did you mean something else?
Yeah sorry that was imprecise. I meant the struct page/folio layout implementation details (which may or may not be related to the page allocator). > Jason >
