On 24 Sep 2025, at 6:55, David Hildenbrand wrote: > On 18.09.25 04:49, Zi Yan wrote: >> On 16 Sep 2025, at 8:21, Balbir Singh wrote: >> >>> Add routines to support allocation of large order zone device folios >>> and helper functions for zone device folios, to check if a folio is >>> device private and helpers for setting zone device data. >>> >>> When large folios are used, the existing page_free() callback in >>> pgmap is called when the folio is freed, this is true for both >>> PAGE_SIZE and higher order pages. >>> >>> Zone device private large folios do not support deferred split and >>> scan like normal THP folios. >>> >>> Signed-off-by: Balbir Singh <[email protected]> >>> Cc: David Hildenbrand <[email protected]> >>> Cc: Zi Yan <[email protected]> >>> Cc: Joshua Hahn <[email protected]> >>> Cc: Rakie Kim <[email protected]> >>> Cc: Byungchul Park <[email protected]> >>> Cc: Gregory Price <[email protected]> >>> Cc: Ying Huang <[email protected]> >>> Cc: Alistair Popple <[email protected]> >>> Cc: Oscar Salvador <[email protected]> >>> Cc: Lorenzo Stoakes <[email protected]> >>> Cc: Baolin Wang <[email protected]> >>> Cc: "Liam R. Howlett" <[email protected]> >>> Cc: Nico Pache <[email protected]> >>> Cc: Ryan Roberts <[email protected]> >>> Cc: Dev Jain <[email protected]> >>> Cc: Barry Song <[email protected]> >>> Cc: Lyude Paul <[email protected]> >>> Cc: Danilo Krummrich <[email protected]> >>> Cc: David Airlie <[email protected]> >>> Cc: Simona Vetter <[email protected]> >>> Cc: Ralph Campbell <[email protected]> >>> Cc: Mika Penttilä <[email protected]> >>> Cc: Matthew Brost <[email protected]> >>> Cc: Francois Dugast <[email protected]> >>> --- >>> include/linux/memremap.h | 10 +++++++++- >>> mm/memremap.c | 34 +++++++++++++++++++++------------- >>> mm/rmap.c | 6 +++++- >>> 3 files changed, 35 insertions(+), 15 deletions(-) >>> >>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h >>> index e5951ba12a28..9c20327c2be5 100644 >>> --- a/include/linux/memremap.h >>> +++ b/include/linux/memremap.h >>> @@ -206,7 +206,7 @@ static inline bool is_fsdax_page(const struct page >>> *page) >>> } >>> >>> #ifdef CONFIG_ZONE_DEVICE >>> -void zone_device_page_init(struct page *page); >>> +void zone_device_folio_init(struct folio *folio, unsigned int order); >>> void *memremap_pages(struct dev_pagemap *pgmap, int nid); >>> void memunmap_pages(struct dev_pagemap *pgmap); >>> void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap); >>> @@ -215,6 +215,14 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn); >>> bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn); >>> >>> unsigned long memremap_compat_align(void); >>> + >>> +static inline void zone_device_page_init(struct page *page) >>> +{ >>> + struct folio *folio = page_folio(page); >>> + >>> + zone_device_folio_init(folio, 0); >> >> I assume it is for legacy code, where only non-compound page exists? >> >> It seems that you assume @page is always order-0, but there is no check >> for it. Adding VM_WARN_ON_ONCE_FOLIO(folio_order(folio) != 0, folio) >> above it would be useful to detect misuse. >> >>> +} >>> + >>> #else >>> static inline void *devm_memremap_pages(struct device *dev, >>> struct dev_pagemap *pgmap) >>> diff --git a/mm/memremap.c b/mm/memremap.c >>> index 46cb1b0b6f72..a8481ebf94cc 100644 >>> --- a/mm/memremap.c >>> +++ b/mm/memremap.c >>> @@ -416,20 +416,19 @@ EXPORT_SYMBOL_GPL(get_dev_pagemap); >>> void free_zone_device_folio(struct folio *folio) >>> { >>> struct dev_pagemap *pgmap = folio->pgmap; >>> + unsigned long nr = folio_nr_pages(folio); >>> + int i; >>> >>> if (WARN_ON_ONCE(!pgmap)) >>> return; >>> >>> mem_cgroup_uncharge(folio); >>> >>> - /* >>> - * Note: we don't expect anonymous compound pages yet. Once supported >>> - * and we could PTE-map them similar to THP, we'd have to clear >>> - * PG_anon_exclusive on all tail pages. >>> - */ >>> if (folio_test_anon(folio)) { >>> - VM_BUG_ON_FOLIO(folio_test_large(folio), folio); >>> - __ClearPageAnonExclusive(folio_page(folio, 0)); >>> + for (i = 0; i < nr; i++) >>> + __ClearPageAnonExclusive(folio_page(folio, i)); >>> + } else { >>> + VM_WARN_ON_ONCE(folio_test_large(folio)); >>> } >>> >>> /* >>> @@ -456,8 +455,8 @@ void free_zone_device_folio(struct folio *folio) >>> case MEMORY_DEVICE_COHERENT: >>> if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->page_free)) >>> break; >>> - pgmap->ops->page_free(folio_page(folio, 0)); >>> - put_dev_pagemap(pgmap); >>> + pgmap->ops->page_free(&folio->page); >>> + percpu_ref_put_many(&folio->pgmap->ref, nr); >>> break; >>> >>> case MEMORY_DEVICE_GENERIC: >>> @@ -480,14 +479,23 @@ void free_zone_device_folio(struct folio *folio) >>> } >>> } >>> >>> -void zone_device_page_init(struct page *page) >>> +void zone_device_folio_init(struct folio *folio, unsigned int order) >>> { >>> + struct page *page = folio_page(folio, 0); >> >> It is strange to see a folio is converted back to page in >> a function called zone_device_folio_init(). >> >>> + >>> + VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES); >>> + >>> /* >>> * Drivers shouldn't be allocating pages after calling >>> * memunmap_pages(). >>> */ >>> - WARN_ON_ONCE(!percpu_ref_tryget_live(&page_pgmap(page)->ref)); >>> - set_page_count(page, 1); >>> + WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << >>> order)); >>> + folio_set_count(folio, 1); >>> lock_page(page); >>> + >>> + if (order > 1) { >>> + prep_compound_page(page, order); >>> + folio_set_large_rmappable(folio); >>> + } >> >> OK, so basically, @folio is not a compound page yet when >> zone_device_folio_init() >> is called. >> >> I feel that your zone_device_page_init() and zone_device_folio_init() >> implementations are inverse. They should follow the same pattern >> as __alloc_pages_noprof() and __folio_alloc_noprof(), where >> zone_device_page_init() does the actual initialization and >> zone_device_folio_init() just convert a page to folio. >> >> Something like: >> >> void zone_device_page_init(struct page *page, unsigned int order) >> { >> VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES); >> >> /* >> * Drivers shouldn't be allocating pages after calling >> * memunmap_pages(). >> */ >> >> WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << >> order)); >> >> /* >> * anonymous folio does not support order-1, high order file-backed >> folio >> * is not supported at all. >> */ >> VM_WARN_ON_ONCE(order == 1); >> >> if (order > 1) >> prep_compound_page(page, order); >> >> /* page has to be compound head here */ >> set_page_count(page, 1); >> lock_page(page); >> } >> >> void zone_device_folio_init(struct folio *folio, unsigned int order) >> { >> struct page *page = folio_page(folio, 0); >> >> zone_device_page_init(page, order); >> page_rmappable_folio(page); >> } >> >> Or >> >> struct folio *zone_device_folio_init(struct page *page, unsigned int order) >> { >> zone_device_page_init(page, order); >> return page_rmappable_folio(page); >> } > > I think the problem is that it will all be weird once we dynamically allocate > "struct folio". > > I have not yet a clear understanding on how that would really work. > > For example, should it be pgmap->ops->page_folio() ? > > Who allocates the folio? Do we allocate all order-0 folios initially, to then > merge them when constructing large folios? How do we manage the "struct > folio" during such merging splitting?
Right. Either we would waste memory by simply concatenating all “struct folio” and putting paddings at the end, or we would free tail “struct folio” first, then allocate tail “struct page”. Both are painful and do not match core mm’s memdesc pattern, where “struct folio” is allocated when caller is asking for a folio. If “struct folio” is always allocated, there is no difference between “struct folio” and “struct page”. > > With that in mind, I don't really know what the proper interface should be > today. > > > zone_device_folio_init(struct page *page, unsigned int order) > > looks cleaner, agreed. > >> >> >> Then, it comes to free_zone_device_folio() above, >> I feel that pgmap->ops->page_free() should take an additional order >> parameter to free a compound page like free_frozen_pages(). >> > > IIRC free_frozen_pages() does not operate on compound pages. If we know that > we are operating on a compound page (or single page) then passing in the page > (or better the folio) should work. free_pages_prepare() in __free_frozen_pages(), called by free_frozen_pages(), checks if compound_order(page) matches the given order, in case folio field corrupts. I suppose it is useful. But I do not have a strong opinion about this one. Best Regards, Yan, Zi
