mm/migrate_device: add THP splitting during migration

Implement migrate_vma_split_pages() to handle THP splitting during the
migration process when destination cannot allocate compound pages.

This addresses the common scenario where migrate_vma_setup() succeeds with
MIGRATE_PFN_COMPOUND pages, but the destination device cannot allocate
large pages during the migration phase.

Key changes:
- migrate_vma_split_pages(): Split already-isolated pages during migration
- Enhanced folio_split() and __split_unmapped_folio() with isolated
  parameter to avoid redundant unmap/remap operations

This provides a fallback mechansim to ensure migration succeeds even when
large page allocation fails at the destination.

[matthew.brost@intel.com: add THP splitting during migration]
  Link: https://lkml.kernel.org/r/20251120230825.181072-2-matthew.brost@intel.com
Link: https://lkml.kernel.org/r/20251001065707.920170-12-balbirs@nvidia.com
Signed-off-by: Balbir Singh <balbirs@nvidia.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.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: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 76 insertions, 11 deletions

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index a0a315f3572a..ab373fd38961 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -309,6 +309,25 @@ again:
 			    pgmap->owner != migrate->pgmap_owner)
 				goto next;
 
+			folio = page_folio(page);
+			if (folio_test_large(folio)) {
+				int ret;
+
+				arch_leave_lazy_mmu_mode();
+				pte_unmap_unlock(ptep, ptl);
+				ret = migrate_vma_split_folio(folio,
+							  migrate->fault_page);
+
+				if (ret) {
+					if (unmapped)
+						flush_tlb_range(walk->vma, start, end);
+
+					return migrate_vma_collect_skip(addr, end, walk);
+				}
+
+				goto again;
+			}
+
 			mpfn = migrate_pfn(page_to_pfn(page)) |
 					MIGRATE_PFN_MIGRATE;
 			if (is_writable_device_private_entry(entry))
@@ -885,6 +904,29 @@ abort:
 		src[i] &= ~MIGRATE_PFN_MIGRATE;
 	return 0;
 }
+
+static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
+					    unsigned long idx, unsigned long addr,
+					    struct folio *folio)
+{
+	unsigned long i;
+	unsigned long pfn;
+	unsigned long flags;
+	int ret = 0;
+
+	folio_get(folio);
+	split_huge_pmd_address(migrate->vma, addr, true);
+	ret = __split_huge_page_to_list_to_order(folio_page(folio, 0), NULL,
+							0, true);
+	if (ret)
+		return ret;
+	migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND;
+	flags = migrate->src[idx] & ((1UL << MIGRATE_PFN_SHIFT) - 1);
+	pfn = migrate->src[idx] >> MIGRATE_PFN_SHIFT;
+	for (i = 1; i < HPAGE_PMD_NR; i++)
+		migrate->src[i+idx] = migrate_pfn(pfn + i) | flags;
+	return ret;
+}
 #else /* !CONFIG_ARCH_ENABLE_THP_MIGRATION */
 static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
 					 unsigned long addr,
@@ -894,6 +936,13 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
 {
 	return 0;
 }
+
+static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
+					    unsigned long idx, unsigned long addr,
+					    struct folio *folio)
+{
+	return 0;
+}
 #endif
 
 static unsigned long migrate_vma_nr_pages(unsigned long *src)
@@ -1055,8 +1104,9 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 				struct migrate_vma *migrate)
 {
 	struct mmu_notifier_range range;
-	unsigned long i;
+	unsigned long i, j;
 	bool notified = false;
+	unsigned long addr;
 
 	for (i = 0; i < npages; ) {
 		struct page *newpage = migrate_pfn_to_page(dst_pfns[i]);
@@ -1098,12 +1148,16 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 				(!(dst_pfns[i] & MIGRATE_PFN_COMPOUND))) {
 				nr = migrate_vma_nr_pages(&src_pfns[i]);
 				src_pfns[i] &= ~MIGRATE_PFN_COMPOUND;
-				src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
-				goto next;
+			} else {
+				nr = 1;
 			}
 
-			migrate_vma_insert_page(migrate, addr, &dst_pfns[i],
-						&src_pfns[i]);
+			for (j = 0; j < nr && i + j < npages; j++) {
+				src_pfns[i+j] |= MIGRATE_PFN_MIGRATE;
+				migrate_vma_insert_page(migrate,
+					addr + j * PAGE_SIZE,
+					&dst_pfns[i+j], &src_pfns[i+j]);
+			}
 			goto next;
 		}
 
@@ -1125,7 +1179,13 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 							 MIGRATE_PFN_COMPOUND);
 					goto next;
 				}
-				src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
+				nr = 1 << folio_order(folio);
+				addr = migrate->start + i * PAGE_SIZE;
+				if (migrate_vma_split_unmapped_folio(migrate, i, addr, folio)) {
+					src_pfns[i] &= ~(MIGRATE_PFN_MIGRATE |
+							 MIGRATE_PFN_COMPOUND);
+					goto next;
+				}
 			} else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) &&
 				(dst_pfns[i] & MIGRATE_PFN_COMPOUND) &&
 				!(src_pfns[i] & MIGRATE_PFN_COMPOUND)) {
@@ -1161,11 +1221,16 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 
 		if (migrate && migrate->fault_page == page)
 			extra_cnt = 1;
-		r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
-		if (r)
-			src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
-		else
-			folio_migrate_flags(newfolio, folio);
+		for (j = 0; j < nr && i + j < npages; j++) {
+			folio = page_folio(migrate_pfn_to_page(src_pfns[i+j]));
+			newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j]));
+
+			r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
+			if (r)
+				src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE;
+			else
+				folio_migrate_flags(newfolio, folio);
+		}
 next:
 		i += nr;
 	}