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?
> + }
> +}
> +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
