Re: [v5 02/15] mm/huge_memory: add device-private THP support to PMD operations

[David Hildenbrand <david@redhat.com>]

On 08.09.25 02:04, Balbir Singh 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: 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>
> 
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> ---
>   include/linux/swapops.h | 27 +++++++++++++++++++
>   mm/huge_memory.c        | 60 ++++++++++++++++++++++++++++++++++++-----
>   mm/pgtable-generic.c    |  6 +++++
>   3 files changed, 86 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 64ea151a7ae3..59c5889a4d54 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -594,6 +594,33 @@ 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;
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 26cedfcd7418..2af74e09b279 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1703,8 +1703,11 @@ 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)) {
> +		VM_WARN_ON(!is_pmd_migration_entry(pmd) &&
> +				!is_pmd_device_private_entry(pmd));
> +

Wrong indentation.

> +		if (is_migration_entry(entry) &&
> +			!is_readable_migration_entry(entry)) {

Dito.

Wonder if we want to be more explicit.

if (is_readable_migration_entry(enrty) ||
     is_readable_exclusive_migration_entry)) {


>   			entry = make_readable_migration_entry(
>   							swp_offset(entry));
>   			pmd = swp_entry_to_pmd(entry);
> @@ -1713,7 +1716,37 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>   			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));

Put this on a single line.

> +				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.
> +			 */
> +			ret = folio_try_dup_anon_rmap_pmd(src_folio,
> +							  &src_folio->page,
> +							  dst_vma, src_vma);
> +			VM_WARN_ON(ret);

Please just drop the ret + VM_WARN_ON here, like we did in the PTE case.

>   		}
> +
>   		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 +2244,17 @@ 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_migration_entry(orig_pmd) ||
> +				is_pmd_device_private_entry(orig_pmd)) {


Indentation ...

>   			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 +2274,12 @@ 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);
> +		}
> +
>   		spin_unlock(ptl);
>   		if (flush_needed)
>   			tlb_remove_page_size(tlb, &folio->page, HPAGE_PMD_SIZE);
> @@ -2367,7 +2408,8 @@ 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_migration_entry(*pmd) &&
> +			   !folio_is_device_private(folio));
>   		if (is_writable_migration_entry(entry)) {
>   			/*
>   			 * A protection check is difficult so
> @@ -2380,6 +2422,10 @@ 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..604e8206a2ec 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -292,6 +292,12 @@ pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
>   		*pmdvalp = pmdval;
>   	if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
>   		goto nomap;
> +	if (is_swap_pmd(pmdval)) {
> +		swp_entry_t entry = pmd_to_swp_entry(pmdval);
> +
> +		if (is_device_private_entry(entry))
> +			goto nomap;
> +	}

Couldn't we do here

if (!pmd_present(pmdval))
	goto nomap;

To replace the original pmd_none() .. check.

A page table must always be present IIRC.

-- 
Cheers

David / dhildenb