Re: [v6 04/15] mm/huge_memory: implement device-private THP splitting

[Zi Yan <ziy@nvidia.com>]

On 16 Sep 2025, at 8:21, Balbir Singh wrote:

> Add support for splitting device-private THP folios, enabling fallback
> to smaller page sizes when large page allocation or migration fails.
>
> Key changes:
> - split_huge_pmd(): Handle device-private PMD entries during splitting
> - Preserve RMAP_EXCLUSIVE semantics for anonymous exclusive folios
> - Skip RMP_USE_SHARED_ZEROPAGE for device-private entries as they
>   don't support shared zero page semantics
>
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> 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/huge_memory.c | 138 +++++++++++++++++++++++++++++++++--------------
>  1 file changed, 98 insertions(+), 40 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 78166db72f4d..5291ee155a02 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2872,16 +2872,18 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	struct page *page;
>  	pgtable_t pgtable;
>  	pmd_t old_pmd, _pmd;
> -	bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
> -	bool anon_exclusive = false, dirty = false;
> +	bool soft_dirty, uffd_wp = false, young = false, write = false;
> +	bool anon_exclusive = false, dirty = false, present = false;
>  	unsigned long addr;
>  	pte_t *pte;
>  	int i;
> +	swp_entry_t swp_entry;
>
>  	VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
>  	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
>  	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
> -	VM_BUG_ON(!is_pmd_migration_entry(*pmd) && !pmd_trans_huge(*pmd));
> +
> +	VM_WARN_ON(!is_pmd_non_present_folio_entry(*pmd) && !pmd_trans_huge(*pmd));
>
>  	count_vm_event(THP_SPLIT_PMD);
>
> @@ -2929,20 +2931,47 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  		return __split_huge_zero_page_pmd(vma, haddr, pmd);
>  	}
>
> -	pmd_migration = is_pmd_migration_entry(*pmd);
> -	if (unlikely(pmd_migration)) {
> -		swp_entry_t entry;
>
> +	present = pmd_present(*pmd);
> +	if (is_pmd_migration_entry(*pmd)) {
>  		old_pmd = *pmd;
> -		entry = pmd_to_swp_entry(old_pmd);
> -		page = pfn_swap_entry_to_page(entry);
> -		write = is_writable_migration_entry(entry);
> +		swp_entry = pmd_to_swp_entry(old_pmd);
> +		page = pfn_swap_entry_to_page(swp_entry);
> +		folio = page_folio(page);
> +
> +		soft_dirty = pmd_swp_soft_dirty(old_pmd);
> +		uffd_wp = pmd_swp_uffd_wp(old_pmd);
> +
> +		write = is_writable_migration_entry(swp_entry);
>  		if (PageAnon(page))
> -			anon_exclusive = is_readable_exclusive_migration_entry(entry);
> -		young = is_migration_entry_young(entry);
> -		dirty = is_migration_entry_dirty(entry);
> +			anon_exclusive = is_readable_exclusive_migration_entry(swp_entry);
> +		young = is_migration_entry_young(swp_entry);
> +		dirty = is_migration_entry_dirty(swp_entry);
> +	} else if (is_pmd_device_private_entry(*pmd)) {
> +		old_pmd = *pmd;
> +		swp_entry = pmd_to_swp_entry(old_pmd);
> +		page = pfn_swap_entry_to_page(swp_entry);
> +		folio = page_folio(page);
> +
>  		soft_dirty = pmd_swp_soft_dirty(old_pmd);
>  		uffd_wp = pmd_swp_uffd_wp(old_pmd);
> +
> +		write = is_writable_device_private_entry(swp_entry);
> +		anon_exclusive = PageAnonExclusive(page);
> +
> +		if (freeze && anon_exclusive &&
> +		    folio_try_share_anon_rmap_pmd(folio, page))
> +			freeze = false;

Why is it OK to change the freeze request? OK, it is replicating
the code for present PMD folios. Either add a comment to point
to the explanation in the comment below, or move
“if (is_pmd_device_private_entry(*pmd))“ branch in the else below
to deduplicate this code.

> +		if (!freeze) {
> +			rmap_t rmap_flags = RMAP_NONE;
> +
> +			folio_ref_add(folio, HPAGE_PMD_NR - 1);
> +			if (anon_exclusive)
> +				rmap_flags |= RMAP_EXCLUSIVE;
> +
> +			folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
> +						 vma, haddr, rmap_flags);
> +		}
>  	} else {
>  		/*
>  		 * Up to this point the pmd is present and huge and userland has
> @@ -3026,32 +3055,57 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	 * Note that NUMA hinting access restrictions are not transferred to
>  	 * avoid any possibility of altering permissions across VMAs.
>  	 */
> -	if (freeze || pmd_migration) {
> -		for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
> -			pte_t entry;
> -			swp_entry_t swp_entry;
> -
> -			if (write)
> -				swp_entry = make_writable_migration_entry(
> -							page_to_pfn(page + i));
> -			else if (anon_exclusive)
> -				swp_entry = make_readable_exclusive_migration_entry(
> -							page_to_pfn(page + i));
> -			else
> -				swp_entry = make_readable_migration_entry(
> -							page_to_pfn(page + i));
> -			if (young)
> -				swp_entry = make_migration_entry_young(swp_entry);
> -			if (dirty)
> -				swp_entry = make_migration_entry_dirty(swp_entry);
> -			entry = swp_entry_to_pte(swp_entry);
> -			if (soft_dirty)
> -				entry = pte_swp_mksoft_dirty(entry);
> -			if (uffd_wp)
> -				entry = pte_swp_mkuffd_wp(entry);
> +	if (freeze || !present) {
> +		pte_t entry;
>
> -			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
> -			set_pte_at(mm, addr, pte + i, entry);
> +		if (freeze || is_migration_entry(swp_entry)) {
>
<snip>
> +		} else {
<snip>
>  		}
>  	} else {
>  		pte_t entry;

David already pointed this out in v5. It can be done such as:

if (freeze || pmd_migration) {
...
} else if (is_pmd_device_private_entry(old_pmd)) {
...
} else {
/* for present, non freeze case */
}

> @@ -3076,7 +3130,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	}
>  	pte_unmap(pte);
>
> -	if (!pmd_migration)
> +	if (!is_pmd_migration_entry(*pmd))
>  		folio_remove_rmap_pmd(folio, page, vma);
>  	if (freeze)
>  		put_page(page);
> @@ -3089,7 +3143,7 @@ void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
>  			   pmd_t *pmd, bool freeze)
>  {
>  	VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
> -	if (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd))
> +	if (pmd_trans_huge(*pmd) || is_pmd_non_present_folio_entry(*pmd))
>  		__split_huge_pmd_locked(vma, pmd, address, freeze);
>  }
>
> @@ -3268,6 +3322,9 @@ static void lru_add_split_folio(struct folio *folio, struct folio *new_folio,
>  	VM_BUG_ON_FOLIO(folio_test_lru(new_folio), folio);
>  	lockdep_assert_held(&lruvec->lru_lock);
>
> +	if (folio_is_device_private(folio))
> +		return;
> +
>  	if (list) {
>  		/* page reclaim is reclaiming a huge page */
>  		VM_WARN_ON(folio_test_lru(folio));
> @@ -3885,8 +3942,9 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	if (nr_shmem_dropped)
>  		shmem_uncharge(mapping->host, nr_shmem_dropped);
>
> -	if (!ret && is_anon)
> +	if (!ret && is_anon && !folio_is_device_private(folio))
>  		remap_flags = RMP_USE_SHARED_ZEROPAGE;
> +

You should remove this and add

if (folio_is_device_private(folio))
	return false;

in try_to_map_unused_to_zeropage(). Otherwise, no one would know
device private folios need to be excluded from mapping unused to
zero page.

>  	remap_page(folio, 1 << order, remap_flags);
>
>  	/*
> -- 
> 2.50.1


Best Regards,
Yan, Zi