Re: [v6 03/15] mm/rmap: extend rmap and migration support device-private entries

[Zi Yan <ziy@nvidia.com>]

On 16 Sep 2025, at 8:21, Balbir Singh 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.
>
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> Reviewed-by: SeongJae Park <sj@kernel.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>
> ---
>  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             | 21 +++++++++++++++++----
>  5 files changed, 61 insertions(+), 10 deletions(-)
>
> diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
> index 998c5180a603..eda4de553611 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 + PAGE_SIZE);

This should be HPAGE_PMD_SIZE (it is guarded in CONFIG_TRANSPARENT_HUGEPAGE,
so HPAGE_PMD_SIZE will not trigger a build bug like the one below).

> +	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 a5e4c2aef191..78166db72f4d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4637,7 +4637,10 @@ 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);
> @@ -4687,6 +4690,17 @@ 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..3bf0fbe05cc2 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 + PAGE_SIZE);

This should be HPAGE_PMD_SIZE (or PMD_SIZE, since the code is not compiled
out when CONFIG_TRANSPARENT_HUGEPAGE is not selected and HPAGE_PMD_SIZE
will cause a build bug when CONFIG_PGTABLE_HAS_HUGE_LEAVES is not selected).

>  		} else {
>  			/* unexpected pmd-mapped page? */
>  			WARN_ON_ONCE(1);
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index e981a1a292d2..159953c590cc 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;
> +			}
> +
>  			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 9a2aabfaea6f..080fc4048431 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1063,9 +1063,11 @@ 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))

It is better to add a similar comment as the one above !pte_present().
Something like:
PFN swap PMDs, such as ...


> +			if (!pmd_present(entry))
> +				continue;
> +			if (!pmd_dirty(entry) && !pmd_write(entry))
>  				continue;
>
>  			flush_cache_range(vma, address,
> @@ -2330,6 +2332,11 @@ 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) {
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +			unsigned long pfn;
> +			pmd_t pmdval;
> +#endif
> +

This looks ugly. IIRC, we now can put variable definition in the middle.
Maybe for this case, these two can be moved to the below ifdef region.

>  			if (flags & TTU_SPLIT_HUGE_PMD) {
>  				split_huge_pmd_locked(vma, pvmw.address,
>  						      pvmw.pmd, true);
> @@ -2338,8 +2345,14 @@ 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);
>
> -- 
> 2.50.1

Otherwise, LGTM. Acked-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi