On 1/12/26 11:16, Matthew Brost wrote: > On Mon, Jan 12, 2026 at 11:44:15AM +1100, Balbir Singh wrote: >> On 1/12/26 06:55, Francois Dugast wrote: >>> From: Matthew Brost <[email protected]> >>> >>> Add free_zone_device_folio_prepare(), a helper that restores large >>> ZONE_DEVICE folios to a sane, initial state before freeing them. >>> >>> Compound ZONE_DEVICE folios overwrite per-page state (e.g. pgmap and >>> compound metadata). Before returning such pages to the device pgmap >>> allocator, each constituent page must be reset to a standalone >>> ZONE_DEVICE folio with a valid pgmap and no compound state. >>> >>> Use this helper prior to folio_free() for device-private and >>> device-coherent folios to ensure consistent device page state for >>> subsequent allocations. >>> >>> Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private >>> folios") >>> Cc: Zi Yan <[email protected]> >>> Cc: David Hildenbrand <[email protected]> >>> Cc: Oscar Salvador <[email protected]> >>> Cc: Andrew Morton <[email protected]> >>> Cc: Balbir Singh <[email protected]> >>> Cc: Lorenzo Stoakes <[email protected]> >>> Cc: Liam R. Howlett <[email protected]> >>> Cc: Vlastimil Babka <[email protected]> >>> Cc: Mike Rapoport <[email protected]> >>> Cc: Suren Baghdasaryan <[email protected]> >>> Cc: Michal Hocko <[email protected]> >>> Cc: Alistair Popple <[email protected]> >>> Cc: [email protected] >>> Cc: [email protected] >>> Cc: [email protected] >>> Suggested-by: Alistair Popple <[email protected]> >>> Signed-off-by: Matthew Brost <[email protected]> >>> Signed-off-by: Francois Dugast <[email protected]> >>> --- >>> include/linux/memremap.h | 1 + >>> mm/memremap.c | 55 ++++++++++++++++++++++++++++++++++++++++ >>> 2 files changed, 56 insertions(+) >>> >>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h >>> index 97fcffeb1c1e..88e1d4707296 100644 >>> --- a/include/linux/memremap.h >>> +++ b/include/linux/memremap.h >>> @@ -230,6 +230,7 @@ static inline bool is_fsdax_page(const struct page >>> *page) >>> >>> #ifdef CONFIG_ZONE_DEVICE >>> void zone_device_page_init(struct page *page, unsigned int order); >>> +void free_zone_device_folio_prepare(struct folio *folio); >>> 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); >>> diff --git a/mm/memremap.c b/mm/memremap.c >>> index 39dc4bd190d0..375a61e18858 100644 >>> --- a/mm/memremap.c >>> +++ b/mm/memremap.c >>> @@ -413,6 +413,60 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn) >>> } >>> EXPORT_SYMBOL_GPL(get_dev_pagemap); >>> >>> +/** >>> + * free_zone_device_folio_prepare() - Prepare a ZONE_DEVICE folio for >>> freeing. >>> + * @folio: ZONE_DEVICE folio to prepare for release. >>> + * >>> + * ZONE_DEVICE pages/folios (e.g., device-private memory or fsdax-backed >>> pages) >>> + * can be compound. When freeing a compound ZONE_DEVICE folio, the tail >>> pages >>> + * must be restored to a sane ZONE_DEVICE state before they are released. >>> + * >>> + * This helper: >>> + * - Clears @folio->mapping and, for compound folios, clears each page's >>> + * compound-head state (ClearPageHead()/clear_compound_head()). >>> + * - Resets the compound order metadata (folio_reset_order()) and then >>> + * initializes each constituent page as a standalone ZONE_DEVICE folio: >>> + * * clears ->mapping >>> + * * restores ->pgmap (prep_compound_page() overwrites it) >>> + * * clears ->share (only relevant for fsdax; unused for >>> device-private) >>> + * >>> + * If @folio is order-0, only the mapping is cleared and no further work is >>> + * required. >>> + */ >>> +void free_zone_device_folio_prepare(struct folio *folio) >>> +{ >>> + struct dev_pagemap *pgmap = page_pgmap(&folio->page); >>> + int order, i; >>> + >>> + VM_WARN_ON_FOLIO(!folio_is_zone_device(folio), folio); >>> + >>> + folio->mapping = NULL; >>> + order = folio_order(folio); >>> + if (!order) >>> + return; >>> + >>> + folio_reset_order(folio); >>> + >>> + for (i = 0; i < (1UL << order); i++) { >>> + struct page *page = folio_page(folio, i); >>> + struct folio *new_folio = (struct folio *)page; >>> + >>> + ClearPageHead(page); >>> + clear_compound_head(page); >>> + >>> + new_folio->mapping = NULL; >>> + /* >>> + * Reset pgmap which was over-written by >>> + * prep_compound_page(). >>> + */ >>> + new_folio->pgmap = pgmap; >>> + 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); >> >> Does calling the free_folio() callback on new_folio solve the issue you are >> facing, or is >> that PMD_ORDER more frees than we'd like? >> > > No, calling free_folio() more often doesn’t solve anything—in fact, that > would make my implementation explode. I explained this in detail here [1] > to Zi. > > To recap [1], my memory allocator has no visibility into individual > pages or folios; it is DRM Buddy layered on top of TTM BO. This design > allows VRAM to be allocated or evicted for both traditional GPU > allocations (GEMs) and SVM allocations. >
I assume it is still backed by pages that are ref counted? I suspect you'd need to convert one reference count to PMD_ORDER reference counts to make this change work, or are the references not at page granularity? I followed the code through drm_zdd_pagemap_put() and zdd->refcount seemed like a per folio refcount > Now, to recap the actual issue: if device folios are not split upon free > and are later reallocated with a different order in > zone_device_page_init, the implementation breaks. This problem is not > specific to Xe—Nouveau happens to always allocate at the same order, so > it works by coincidence. Reallocating at a different order is valid > behavior and must be supported. > Agreed > Matt > > [1] > https://patchwork.freedesktop.org/patch/697710/?series=159119&rev=3#comment_1282413 > >>> + } >>> +} >>> +EXPORT_SYMBOL_GPL(free_zone_device_folio_prepare); >>> + >>> void free_zone_device_folio(struct folio *folio) >>> { >>> struct dev_pagemap *pgmap = folio->pgmap; >>> @@ -454,6 +508,7 @@ void free_zone_device_folio(struct folio *folio) >>> case MEMORY_DEVICE_COHERENT: >>> if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->folio_free)) >>> break; >>> + free_zone_device_folio_prepare(folio); >>> pgmap->ops->folio_free(folio, order); >>> percpu_ref_put_many(&folio->pgmap->ref, nr); >>> break; >> >> Balbir
