Re: [v7 03/16] mm/huge_memory: add device-private THP support to PMD operations

[Lance Yang <lance.yang@linux.dev>]

On Wed, Oct 1, 2025 at 4:20 PM Balbir Singh <balbirs@nvidia.com> wrote:
>
> Extend core huge page management functions to handle device-private THP
> entries.  This enables proper handling of large device-private folios in
> fundamental MM operations.
>
> The following functions have been updated:
>
> - copy_huge_pmd(): Handle device-private entries during fork/clone
> - zap_huge_pmd(): Properly free device-private THP during munmap
> - change_huge_pmd(): Support protection changes on device-private THP
> - __pte_offset_map(): Add device-private entry awareness
>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
> Cc: Rakie Kim <rakie.kim@sk.com>
> Cc: Byungchul Park <byungchul@sk.com>
> Cc: Gregory Price <gourry@gourry.net>
> Cc: Ying Huang <ying.huang@linux.alibaba.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Danilo Krummrich <dakr@kernel.org>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Mika Penttilä <mpenttil@redhat.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Francois Dugast <francois.dugast@intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> ---
>  include/linux/swapops.h | 32 +++++++++++++++++++++++
>  mm/huge_memory.c        | 56 ++++++++++++++++++++++++++++++++++-------
>  mm/pgtable-generic.c    |  2 +-
>  3 files changed, 80 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 64ea151a7ae3..2687928a8146 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -594,10 +594,42 @@ static inline int is_pmd_migration_entry(pmd_t pmd)
>  }
>  #endif  /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
>
> +#if defined(CONFIG_ZONE_DEVICE) && defined(CONFIG_ARCH_ENABLE_THP_MIGRATION)
> +
> +/**
> + * is_pmd_device_private_entry() - Check if PMD contains a device private swap entry
> + * @pmd: The PMD to check
> + *
> + * Returns true if the PMD contains a swap entry that represents a device private
> + * page mapping. This is used for zone device private pages that have been
> + * swapped out but still need special handling during various memory management
> + * operations.
> + *
> + * Return: 1 if PMD contains device private entry, 0 otherwise
> + */
> +static inline int is_pmd_device_private_entry(pmd_t pmd)
> +{
> +       return is_swap_pmd(pmd) && is_device_private_entry(pmd_to_swp_entry(pmd));
> +}
> +
> +#else /* CONFIG_ZONE_DEVICE && CONFIG_ARCH_ENABLE_THP_MIGRATION */
> +
> +static inline int is_pmd_device_private_entry(pmd_t pmd)
> +{
> +       return 0;
> +}
> +
> +#endif /* CONFIG_ZONE_DEVICE && CONFIG_ARCH_ENABLE_THP_MIGRATION */
> +
>  static inline int non_swap_entry(swp_entry_t entry)
>  {
>         return swp_type(entry) >= MAX_SWAPFILES;
>  }
>
> +static inline int is_pmd_non_present_folio_entry(pmd_t pmd)
> +{
> +       return is_pmd_migration_entry(pmd) || is_pmd_device_private_entry(pmd);
> +}
> +
>  #endif /* CONFIG_MMU */
>  #endif /* _LINUX_SWAPOPS_H */
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1b81680b4225..8e0a1747762d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1703,17 +1703,45 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>         if (unlikely(is_swap_pmd(pmd))) {
>                 swp_entry_t entry = pmd_to_swp_entry(pmd);
>
> -               VM_BUG_ON(!is_pmd_migration_entry(pmd));
> -               if (!is_readable_migration_entry(entry)) {
> -                       entry = make_readable_migration_entry(
> -                                                       swp_offset(entry));
> +               VM_WARN_ON(!is_pmd_non_present_folio_entry(pmd));
> +
> +               if (is_writable_migration_entry(entry) ||
> +                   is_readable_exclusive_migration_entry(entry)) {
> +                       entry = make_readable_migration_entry(swp_offset(entry));
>                         pmd = swp_entry_to_pmd(entry);
>                         if (pmd_swp_soft_dirty(*src_pmd))
>                                 pmd = pmd_swp_mksoft_dirty(pmd);
>                         if (pmd_swp_uffd_wp(*src_pmd))
>                                 pmd = pmd_swp_mkuffd_wp(pmd);
>                         set_pmd_at(src_mm, addr, src_pmd, pmd);
> +               } else if (is_device_private_entry(entry)) {
> +                       /*
> +                        * For device private entries, since there are no
> +                        * read exclusive entries, writable = !readable
> +                        */
> +                       if (is_writable_device_private_entry(entry)) {
> +                               entry = make_readable_device_private_entry(swp_offset(entry));
> +                               pmd = swp_entry_to_pmd(entry);
> +
> +                               if (pmd_swp_soft_dirty(*src_pmd))
> +                                       pmd = pmd_swp_mksoft_dirty(pmd);
> +                               if (pmd_swp_uffd_wp(*src_pmd))
> +                                       pmd = pmd_swp_mkuffd_wp(pmd);
> +                               set_pmd_at(src_mm, addr, src_pmd, pmd);
> +                       }
> +
> +                       src_folio = pfn_swap_entry_folio(entry);
> +                       VM_WARN_ON(!folio_test_large(src_folio));
> +
> +                       folio_get(src_folio);
> +                       /*
> +                        * folio_try_dup_anon_rmap_pmd does not fail for
> +                        * device private entries.
> +                        */
> +                       folio_try_dup_anon_rmap_pmd(src_folio, &src_folio->page,
> +                                                       dst_vma, src_vma);
>                 }
> +
>                 add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>                 mm_inc_nr_ptes(dst_mm);
>                 pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
> @@ -2211,15 +2239,16 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>                         folio_remove_rmap_pmd(folio, page, vma);
>                         WARN_ON_ONCE(folio_mapcount(folio) < 0);
>                         VM_BUG_ON_PAGE(!PageHead(page), page);
> -               } else if (thp_migration_supported()) {
> +               } else if (is_pmd_non_present_folio_entry(orig_pmd)) {
>                         swp_entry_t entry;
>
> -                       VM_BUG_ON(!is_pmd_migration_entry(orig_pmd));
>                         entry = pmd_to_swp_entry(orig_pmd);
>                         folio = pfn_swap_entry_folio(entry);
>                         flush_needed = 0;
> -               } else
> -                       WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
> +
> +                       if (!thp_migration_supported())
> +                               WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
> +               }
>
>                 if (folio_test_anon(folio)) {
>                         zap_deposited_table(tlb->mm, pmd);
> @@ -2239,6 +2268,12 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>                                 folio_mark_accessed(folio);
>                 }
>
> +               if (folio_is_device_private(folio)) {
> +                       folio_remove_rmap_pmd(folio, &folio->page, vma);
> +                       WARN_ON_ONCE(folio_mapcount(folio) < 0);
> +                       folio_put(folio);
> +               }

IIUC, a device-private THP is always anonymous, right? would it make sense
to move this folio_is_device_private() block inside the folio_test_anon()
check above?

> +
>                 spin_unlock(ptl);
>                 if (flush_needed)
>                         tlb_remove_page_size(tlb, &folio->page, HPAGE_PMD_SIZE);
> @@ -2367,7 +2402,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>                 struct folio *folio = pfn_swap_entry_folio(entry);
>                 pmd_t newpmd;
>
> -               VM_BUG_ON(!is_pmd_migration_entry(*pmd));
> +               VM_WARN_ON(!is_pmd_non_present_folio_entry(*pmd));
>                 if (is_writable_migration_entry(entry)) {
>                         /*
>                          * A protection check is difficult so
> @@ -2380,6 +2415,9 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>                         newpmd = swp_entry_to_pmd(entry);
>                         if (pmd_swp_soft_dirty(*pmd))
>                                 newpmd = pmd_swp_mksoft_dirty(newpmd);
> +               } else if (is_writable_device_private_entry(entry)) {
> +                       entry = make_readable_device_private_entry(swp_offset(entry));
> +                       newpmd = swp_entry_to_pmd(entry);
>                 } else {
>                         newpmd = *pmd;
>                 }
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index 567e2d084071..0c847cdf4fd3 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -290,7 +290,7 @@ pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
>
>         if (pmdvalp)
>                 *pmdvalp = pmdval;
> -       if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
> +       if (unlikely(pmd_none(pmdval) || !pmd_present(pmdval)))
>                 goto nomap;
>         if (unlikely(pmd_trans_huge(pmdval)))
>                 goto nomap;
> --
> 2.51.0
>
>