Re: [v4 05/15] mm/migrate_device: handle partially mapped folios during collection

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

Hi,

On 9/3/25 04:18, Balbir Singh wrote:

> Extend migrate_vma_collect_pmd() to handle partially mapped large
> folios that require splitting before migration can proceed.
>
> During PTE walk in the collection phase, if a large folio is only
> partially mapped in the migration range, it must be split to ensure
> the folio is correctly migrated.
>
> 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: Balbir Singh <balbirs@nvidia.com>
> ---
>  mm/migrate_device.c | 95 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 95 insertions(+)
>
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index e05e14d6eacd..e58c3f9d01c8 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -54,6 +54,54 @@ static int migrate_vma_collect_hole(unsigned long start,
>  	return 0;
>  }
>  
> +/**
> + * migrate_vma_split_folio - Helper function to split a(n) (m)THP folio
> + *
> + * @folio - the folio to split
> + * @fault_page - struct page associated with the fault if any
> + *
> + * Returns 0 on success
> + */
> +static int migrate_vma_split_folio(struct folio *folio,
> +				   struct page *fault_page)
> +{
> +	int ret;
> +	struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL;
> +	struct folio *new_fault_folio = NULL;
> +
> +	if (folio != fault_folio) {
> +		folio_get(folio);
> +		folio_lock(folio);
> +	}
> +
> +	ret = split_folio(folio);
> +	if (ret) {
> +		if (folio != fault_folio) {
> +			folio_unlock(folio);
> +			folio_put(folio);
> +		}
> +		return ret;
> +	}
> +
> +	new_fault_folio = fault_page ? page_folio(fault_page) : NULL;
> +
> +	/*
> +	 * Ensure the lock is held on the correct
> +	 * folio after the split
> +	 */
> +	if (!new_fault_folio) {
> +		folio_unlock(folio);
> +		folio_put(folio);
> +	} else if (folio != new_fault_folio) {
> +		folio_get(new_fault_folio);
> +		folio_lock(new_fault_folio);
> +		folio_unlock(folio);
> +		folio_put(folio);
> +	}
> +
> +	return 0;
> +}
> +
>  static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  				   unsigned long start,
>  				   unsigned long end,
> @@ -136,6 +184,8 @@ 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;
> @@ -147,6 +197,29 @@ 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)) {
> +				int ret;
> +
> +				/*
> +				 * The reason for finding pmd present with 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);
> +				ret = migrate_vma_split_folio(folio,
> +							  migrate->fault_page);
> +
> +				if (ret) {
> +					ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +					goto next;
> +				}
> +
> +				addr = start;
> +				goto again;
> +			}
> +
>  			mpfn = migrate_pfn(page_to_pfn(page)) |
>  					MIGRATE_PFN_MIGRATE;
>  			if (is_writable_device_private_entry(entry))
> @@ -171,6 +244,28 @@ 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)) {
> +				int ret;
> +
> +				/*
> +				 * The reason for finding pmd present with a
> +				 * large folio for the pte is partial unmaps.
> +				 * Split the folio now for the migration to be
> +				 * handled correctly
> +				 */

There are other reasons like vma splits for various reasons.

> +				pte_unmap_unlock(ptep, ptl);
> +				ret = migrate_vma_split_folio(folio,
> +							  migrate->fault_page);
> +
> +				if (ret) {
> +					ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +					goto next;
> +				}
> +
> +				addr = start;
> +				goto again;
> +			}
>  			mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
>  			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
>  		}