Re: [v7 04/16] mm/rmap: extend rmap and migration support device-private entries

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

On Wed, Oct 1, 2025 at 3:25 PM Balbir Singh <balbirs@nvidia.com> wrote:
>
> Add device-private THP support to reverse mapping infrastructure, enabling
> proper handling during migration and walk operations.
>
> The key changes are:
> - add_migration_pmd()/remove_migration_pmd(): Handle device-private
>   entries during folio migration and splitting
> - page_vma_mapped_walk(): Recognize device-private THP entries during
>   VMA traversal operations
>
> This change supports folio splitting and migration operations on
> device-private entries.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> 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>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> Reviewed-by: SeongJae Park <sj@kernel.org>
> ---
>  mm/damon/ops-common.c | 20 +++++++++++++++++---
>  mm/huge_memory.c      | 16 +++++++++++++++-
>  mm/page_idle.c        |  7 +++++--
>  mm/page_vma_mapped.c  |  7 +++++++
>  mm/rmap.c             | 24 ++++++++++++++++++++----
>  5 files changed, 64 insertions(+), 10 deletions(-)
>
> diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
> index 998c5180a603..ac54bf5b2623 100644
> --- a/mm/damon/ops-common.c
> +++ b/mm/damon/ops-common.c
> @@ -75,12 +75,24 @@ void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr
>  void damon_pmdp_mkold(pmd_t *pmd, struct vm_area_struct *vma, unsigned long addr)
>  {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -       struct folio *folio = damon_get_folio(pmd_pfn(pmdp_get(pmd)));
> +       pmd_t pmdval = pmdp_get(pmd);
> +       struct folio *folio;
> +       bool young = false;
> +       unsigned long pfn;
> +
> +       if (likely(pmd_present(pmdval)))
> +               pfn = pmd_pfn(pmdval);
> +       else
> +               pfn = swp_offset_pfn(pmd_to_swp_entry(pmdval));
>
> +       folio = damon_get_folio(pfn);
>         if (!folio)
>                 return;
>
> -       if (pmdp_clear_young_notify(vma, addr, pmd))
> +       if (likely(pmd_present(pmdval)))
> +               young |= pmdp_clear_young_notify(vma, addr, pmd);
> +       young |= mmu_notifier_clear_young(vma->vm_mm, addr, addr + HPAGE_PMD_SIZE);
> +       if (young)
>                 folio_set_young(folio);
>
>         folio_set_idle(folio);
> @@ -203,7 +215,9 @@ static bool damon_folio_young_one(struct folio *folio,
>                                 mmu_notifier_test_young(vma->vm_mm, addr);
>                 } else {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -                       *accessed = pmd_young(pmdp_get(pvmw.pmd)) ||
> +                       pmd_t pmd = pmdp_get(pvmw.pmd);
> +
> +                       *accessed = (pmd_present(pmd) && pmd_young(pmd)) ||
>                                 !folio_test_idle(folio) ||
>                                 mmu_notifier_test_young(vma->vm_mm, addr);
>  #else
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8e0a1747762d..483b8341ce22 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4628,7 +4628,10 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
>                 return 0;
>
>         flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
> -       pmdval = pmdp_invalidate(vma, address, pvmw->pmd);
> +       if (unlikely(!pmd_present(*pvmw->pmd)))
> +               pmdval = pmdp_huge_get_and_clear(vma->vm_mm, address, pvmw->pmd);
> +       else
> +               pmdval = pmdp_invalidate(vma, address, pvmw->pmd);
>
>         /* See folio_try_share_anon_rmap_pmd(): invalidate PMD first. */
>         anon_exclusive = folio_test_anon(folio) && PageAnonExclusive(page);
> @@ -4678,6 +4681,17 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>         entry = pmd_to_swp_entry(*pvmw->pmd);
>         folio_get(folio);
>         pmde = folio_mk_pmd(folio, READ_ONCE(vma->vm_page_prot));
> +
> +       if (folio_is_device_private(folio)) {
> +               if (pmd_write(pmde))
> +                       entry = make_writable_device_private_entry(
> +                                                       page_to_pfn(new));
> +               else
> +                       entry = make_readable_device_private_entry(
> +                                                       page_to_pfn(new));
> +               pmde = swp_entry_to_pmd(entry);
> +       }
> +
>         if (pmd_swp_soft_dirty(*pvmw->pmd))
>                 pmde = pmd_mksoft_dirty(pmde);
>         if (is_writable_migration_entry(entry))
> diff --git a/mm/page_idle.c b/mm/page_idle.c
> index a82b340dc204..d4299de81031 100644
> --- a/mm/page_idle.c
> +++ b/mm/page_idle.c
> @@ -71,8 +71,11 @@ static bool page_idle_clear_pte_refs_one(struct folio *folio,
>                                 referenced |= ptep_test_and_clear_young(vma, addr, pvmw.pte);
>                         referenced |= mmu_notifier_clear_young(vma->vm_mm, addr, addr + PAGE_SIZE);
>                 } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> -                       if (pmdp_clear_young_notify(vma, addr, pvmw.pmd))
> -                               referenced = true;
> +                       pmd_t pmdval = pmdp_get(pvmw.pmd);
> +
> +                       if (likely(pmd_present(pmdval)))
> +                               referenced |= pmdp_clear_young_notify(vma, addr, pvmw.pmd);
> +                       referenced |= mmu_notifier_clear_young(vma->vm_mm, addr, addr + PMD_SIZE);
>                 } else {
>                         /* unexpected pmd-mapped page? */
>                         WARN_ON_ONCE(1);
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index c498a91b6706..137ce27ff68c 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -277,6 +277,13 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>                          * cannot return prematurely, while zap_huge_pmd() has
>                          * cleared *pmd but not decremented compound_mapcount().
>                          */
> +                       swp_entry_t entry = pmd_to_swp_entry(pmde);
> +
> +                       if (is_device_private_entry(entry)) {
> +                               pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> +                               return true;
> +                       }
> +

We could make this simpler:

                        if (is_device_private_entry(pmd_to_swp_entry(pmde))) {
                                pvmw->ptl = pmd_lock(mm, pvmw->pmd);
                                return true;
                        }

Thanks,
Lance

>                         if ((pvmw->flags & PVMW_SYNC) &&
>                             thp_vma_suitable_order(vma, pvmw->address,
>                                                    PMD_ORDER) &&
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 9bab13429975..c3fc30cf3636 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1046,9 +1046,16 @@ static int page_vma_mkclean_one(struct page_vma_mapped_walk *pvmw)
>                 } else {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>                         pmd_t *pmd = pvmw->pmd;
> -                       pmd_t entry;
> +                       pmd_t entry = pmdp_get(pmd);
>
> -                       if (!pmd_dirty(*pmd) && !pmd_write(*pmd))
> +                       /*
> +                        * Please see the comment above (!pte_present).
> +                        * A non present PMD is not writable from a CPU
> +                        * perspective.
> +                        */
> +                       if (!pmd_present(entry))
> +                               continue;
> +                       if (!pmd_dirty(entry) && !pmd_write(entry))
>                                 continue;
>
>                         flush_cache_range(vma, address,
> @@ -2343,6 +2350,9 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>         while (page_vma_mapped_walk(&pvmw)) {
>                 /* PMD-mapped THP migration entry */
>                 if (!pvmw.pte) {
> +                       __maybe_unused unsigned long pfn;
> +                       __maybe_unused pmd_t pmdval;
> +
>                         if (flags & TTU_SPLIT_HUGE_PMD) {
>                                 split_huge_pmd_locked(vma, pvmw.address,
>                                                       pvmw.pmd, true);
> @@ -2351,8 +2361,14 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>                                 break;
>                         }
>  #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> -                       subpage = folio_page(folio,
> -                               pmd_pfn(*pvmw.pmd) - folio_pfn(folio));
> +                       pmdval = pmdp_get(pvmw.pmd);
> +                       if (likely(pmd_present(pmdval)))
> +                               pfn = pmd_pfn(pmdval);
> +                       else
> +                               pfn = swp_offset_pfn(pmd_to_swp_entry(pmdval));
> +
> +                       subpage = folio_page(folio, pfn - folio_pfn(folio));
> +
>                         VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) ||
>                                         !folio_test_pmd_mappable(folio), folio);
>
> --
> 2.51.0
>
>