Re: [v2 02/11] mm/thp: zone_device awareness in THP handling code

["Mika Penttilä" <mpenttil@redhat.com>]

Hi,

On 8/2/25 13:37, Balbir Singh wrote:
> FYI:
>
> I have the following patch on top of my series that seems to make it work
> without requiring the helper to split device private folios
>
I think this looks much better!

> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> ---
>  include/linux/huge_mm.h |  1 -
>  lib/test_hmm.c          | 11 +++++-
>  mm/huge_memory.c        | 76 ++++-------------------------------------
>  mm/migrate_device.c     | 51 +++++++++++++++++++++++++++
>  4 files changed, 67 insertions(+), 72 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 19e7e3b7c2b7..52d8b435950b 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -343,7 +343,6 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
>  		vm_flags_t vm_flags);
>  
>  bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
> -int split_device_private_folio(struct folio *folio);
>  int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>  		unsigned int new_order, bool unmapped);
>  int min_order_for_split(struct folio *folio);
> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> index 341ae2af44ec..444477785882 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -1625,13 +1625,22 @@ static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf)
>  	 * the mirror but here we use it to hold the page for the simulated
>  	 * device memory and that page holds the pointer to the mirror.
>  	 */
> -	rpage = vmf->page->zone_device_data;
> +	rpage = folio_page(page_folio(vmf->page), 0)->zone_device_data;
>  	dmirror = rpage->zone_device_data;
>  
>  	/* FIXME demonstrate how we can adjust migrate range */
>  	order = folio_order(page_folio(vmf->page));
>  	nr = 1 << order;
>  
> +	/*
> +	 * When folios are partially mapped, we can't rely on the folio
> +	 * order of vmf->page as the folio might not be fully split yet
> +	 */
> +	if (vmf->pte) {
> +		order = 0;
> +		nr = 1;
> +	}
> +
>  	/*
>  	 * Consider a per-cpu cache of src and dst pfns, but with
>  	 * large number of cpus that might not scale well.
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1fc1efa219c8..863393dec1f1 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -72,10 +72,6 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
>  					  struct shrink_control *sc);
>  static unsigned long deferred_split_scan(struct shrinker *shrink,
>  					 struct shrink_control *sc);
> -static int __split_unmapped_folio(struct folio *folio, int new_order,
> -		struct page *split_at, struct xa_state *xas,
> -		struct address_space *mapping, bool uniform_split);
> -
>  static bool split_underused_thp = true;
>  
>  static atomic_t huge_zero_refcount;
> @@ -2924,51 +2920,6 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
>  	pmd_populate(mm, pmd, pgtable);
>  }
>  
> -/**
> - * split_huge_device_private_folio - split a huge device private folio into
> - * smaller pages (of order 0), currently used by migrate_device logic to
> - * split folios for pages that are partially mapped
> - *
> - * @folio: the folio to split
> - *
> - * The caller has to hold the folio_lock and a reference via folio_get
> - */
> -int split_device_private_folio(struct folio *folio)
> -{
> -	struct folio *end_folio = folio_next(folio);
> -	struct folio *new_folio;
> -	int ret = 0;
> -
> -	/*
> -	 * Split the folio now. In the case of device
> -	 * private pages, this path is executed when
> -	 * the pmd is split and since freeze is not true
> -	 * it is likely the folio will be deferred_split.
> -	 *
> -	 * With device private pages, deferred splits of
> -	 * folios should be handled here to prevent partial
> -	 * unmaps from causing issues later on in migration
> -	 * and fault handling flows.
> -	 */
> -	folio_ref_freeze(folio, 1 + folio_expected_ref_count(folio));
> -	ret = __split_unmapped_folio(folio, 0, &folio->page, NULL, NULL, true);
> -	VM_WARN_ON(ret);
> -	for (new_folio = folio_next(folio); new_folio != end_folio;
> -					new_folio = folio_next(new_folio)) {
> -		zone_device_private_split_cb(folio, new_folio);
> -		folio_ref_unfreeze(new_folio, 1 + folio_expected_ref_count(
> -								new_folio));
> -	}
> -
> -	/*
> -	 * Mark the end of the folio split for device private THP
> -	 * split
> -	 */
> -	zone_device_private_split_cb(folio, NULL);
> -	folio_ref_unfreeze(folio, 1 + folio_expected_ref_count(folio));
> -	return ret;
> -}
> -
>  static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  		unsigned long haddr, bool freeze)
>  {
> @@ -3064,30 +3015,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  				freeze = false;
>  			if (!freeze) {
>  				rmap_t rmap_flags = RMAP_NONE;
> -				unsigned long addr = haddr;
> -				struct folio *new_folio;
> -				struct folio *end_folio = folio_next(folio);
>  
>  				if (anon_exclusive)
>  					rmap_flags |= RMAP_EXCLUSIVE;
>  
> -				folio_lock(folio);
> -				folio_get(folio);
> -
> -				split_device_private_folio(folio);
> -
> -				for (new_folio = folio_next(folio);
> -					new_folio != end_folio;
> -					new_folio = folio_next(new_folio)) {
> -					addr += PAGE_SIZE;
> -					folio_unlock(new_folio);
> -					folio_add_anon_rmap_ptes(new_folio,
> -						&new_folio->page, 1,
> -						vma, addr, rmap_flags);
> -				}
> -				folio_unlock(folio);
> -				folio_add_anon_rmap_ptes(folio, &folio->page,
> -						1, vma, haddr, rmap_flags);
> +				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);
>  			}
>  		}
>  
> @@ -4065,7 +4001,7 @@ 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;
>  
>  	remap_page(folio, 1 << order, remap_flags);
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 49962ea19109..4264c0290d08 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -248,6 +248,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			 * page table entry. Other special swap entries are not
>  			 * migratable, and we ignore regular swapped page.
>  			 */
> +			struct folio *folio;
> +
>  			entry = pte_to_swp_entry(pte);
>  			if (!is_device_private_entry(entry))
>  				goto next;
> @@ -259,6 +261,55 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			    pgmap->owner != migrate->pgmap_owner)
>  				goto next;
>  
> +			folio = page_folio(page);
> +			if (folio_test_large(folio)) {
> +				struct folio *new_folio;
> +				struct folio *new_fault_folio;
> +
> +				/*
> +				 * The reason for finding pmd present with a
> +				 * device private pte and a large folio for the
> +				 * pte is partial unmaps. Split the folio now
> +				 * for the migration to be handled correctly
> +				 */
> +				pte_unmap_unlock(ptep, ptl);
> +
> +				folio_get(folio);
> +				if (folio != fault_folio)
> +					folio_lock(folio);
> +				if (split_folio(folio)) {
> +					if (folio != fault_folio)
> +						folio_unlock(folio);
> +					ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +					goto next;
> +				}
> +

The nouveau migrate_to_ram handler needs adjustment also if split happens.

> +				/*
> +				 * After the split, get back the extra reference
> +				 * on the fault_page, this reference is checked during
> +				 * folio_migrate_mapping()
> +				 */
> +				if (migrate->fault_page) {
> +					new_fault_folio = page_folio(migrate->fault_page);
> +					folio_get(new_fault_folio);
> +				}
> +
> +				new_folio = page_folio(page);
> +				pfn = page_to_pfn(page);
> +
> +				/*
> +				 * Ensure the lock is held on the correct
> +				 * folio after the split
> +				 */
> +				if (folio != new_folio) {
> +					folio_unlock(folio);
> +					folio_lock(new_folio);
> +				}

Maybe careful not to unlock fault_page ?

> +				folio_put(folio);
> +				addr = start;
> +				goto again;
> +			}
> +
>  			mpfn = migrate_pfn(page_to_pfn(page)) |
>  					MIGRATE_PFN_MIGRATE;
>  			if (is_writable_device_private_entry(entry))

--Mika