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

[Balbir Singh <balbirs@nvidia.com>]

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
+				 */
+				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;
 		}
-- 
2.50.1