[PATCH] mm/migrate: fix mlock state loss when migrating PTE-mapped large folios

[Wandun Chen <chenwandun1@gmail.com>]

From: Wandun Chen <chenwandun@lixiang.com>

try_to_migrate() clears PG_mlocked when unmapping the source folio.
For PTE-mapped large folios, remove_migration_pte() restores one PTE at
a time, but the rmap helpers only restore mlock for full-folio mappings.
This leaves the destination PTE-mapped large folio without PG_mlocked.

Restore mlock once all PTEs of the destination folio are restored under
a single page table lock.

Fixes: dc68badcede4 ("mm: mlock: update mlock_pte_range to handle large folio")
Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
---
 mm/migrate.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 94033c222e04..140e291d4587 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -353,6 +353,7 @@ static bool remove_migration_pte(struct folio *folio,
 {
 	struct rmap_walk_arg *rmap_walk_arg = arg;
 	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
+	unsigned long nr_ptes = 0;
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		rmap_t rmap_flags = RMAP_NONE;
@@ -446,9 +447,16 @@ static bool remove_migration_pte(struct folio *folio,
 			else
 				folio_add_file_rmap_pte(folio, new, vma);
 			set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
+
+			nr_ptes++;
 		}
-		if (READ_ONCE(vma->vm_flags) & VM_LOCKED)
+		if (READ_ONCE(vma->vm_flags) & VM_LOCKED) {
+			if (folio_test_large(folio) &&
+			    nr_ptes == folio_nr_pages(folio) &&
+			    !(pvmw.flags & PVMW_PGTABLE_CROSSED))
+				mlock_vma_folio(folio, vma);
 			mlock_drain_local();
+		}
 
 		trace_remove_migration_pte(pvmw.address, pte_val(pte),
 					   compound_order(new));
-- 
2.43.0