[PATCH v1 1/2] mm/migrate: remove MIGRATEPAGE_UNMAP

[David Hildenbrand <david@redhat.com>]

migrate_folio_unmap() is the only user of MIGRATEPAGE_UNMAP. We want to
remove MIGRATEPAGE_* completely.

It's rather weird to have a generic MIGRATEPAGE_UNMAP, documented to be
returned from address-space callbacks, when it's only used for an
internal helper.

Let's start by having only a single "success" return value for
migrate_folio_unmap() -- 0 -- by moving the "folio was already freed"
check into the single caller.

There is a remaining comment for PG_isolated, which we renamed to
PG_movable_ops_isolated recently and forgot to update.

While we might still run into that case with zsmalloc, it's something we
want to get rid of soon. So let's just focus that optimization on real
folios only for now by excluding movable_ops pages. Note that concurrent
freeing can happen at any time and this "already freed" check is not
relevant for correctness.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/migrate.h |  1 -
 mm/migrate.c            | 40 ++++++++++++++++++++--------------------
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index acadd41e0b5cf..40f2b5a37efbb 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -18,7 +18,6 @@ struct migration_target_control;
  * - zero on page migration success;
  */
 #define MIGRATEPAGE_SUCCESS		0
-#define MIGRATEPAGE_UNMAP		1
 
 /**
  * struct movable_operations - Driver page migration
diff --git a/mm/migrate.c b/mm/migrate.c
index 425401b2d4e14..e9dacf1028dc7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1176,16 +1176,6 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 	bool locked = false;
 	bool dst_locked = false;
 
-	if (folio_ref_count(src) == 1) {
-		/* Folio was freed from under us. So we are done. */
-		folio_clear_active(src);
-		folio_clear_unevictable(src);
-		/* free_pages_prepare() will clear PG_isolated. */
-		list_del(&src->lru);
-		migrate_folio_done(src, reason);
-		return MIGRATEPAGE_SUCCESS;
-	}
-
 	dst = get_new_folio(src, private);
 	if (!dst)
 		return -ENOMEM;
@@ -1275,7 +1265,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 
 	if (unlikely(page_has_movable_ops(&src->page))) {
 		__migrate_folio_record(dst, old_page_state, anon_vma);
-		return MIGRATEPAGE_UNMAP;
+		return 0;
 	}
 
 	/*
@@ -1305,7 +1295,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 
 	if (!folio_mapped(src)) {
 		__migrate_folio_record(dst, old_page_state, anon_vma);
-		return MIGRATEPAGE_UNMAP;
+		return 0;
 	}
 
 out:
@@ -1848,14 +1838,28 @@ static int migrate_pages_batch(struct list_head *from,
 				continue;
 			}
 
+			/*
+			 * If we are holding the last folio reference, the folio
+			 * was freed from under us, so just drop our reference.
+			 */
+			if (likely(!page_has_movable_ops(&folio->page)) &&
+			    folio_ref_count(folio) == 1) {
+				folio_clear_active(folio);
+				folio_clear_unevictable(folio);
+				list_del(&folio->lru);
+				migrate_folio_done(folio, reason);
+				stats->nr_succeeded += nr_pages;
+				stats->nr_thp_succeeded += is_thp;
+				continue;
+			}
+
 			rc = migrate_folio_unmap(get_new_folio, put_new_folio,
 					private, folio, &dst, mode, reason,
 					ret_folios);
 			/*
 			 * The rules are:
-			 *	Success: folio will be freed
-			 *	Unmap: folio will be put on unmap_folios list,
-			 *	       dst folio put on dst_folios list
+			 *	0: folio will be put on unmap_folios list,
+			 *	   dst folio put on dst_folios list
 			 *	-EAGAIN: stay on the from list
 			 *	-ENOMEM: stay on the from list
 			 *	Other errno: put on ret_folios list
@@ -1905,11 +1909,7 @@ static int migrate_pages_batch(struct list_head *from,
 				thp_retry += is_thp;
 				nr_retry_pages += nr_pages;
 				break;
-			case MIGRATEPAGE_SUCCESS:
-				stats->nr_succeeded += nr_pages;
-				stats->nr_thp_succeeded += is_thp;
-				break;
-			case MIGRATEPAGE_UNMAP:
+			case 0:
 				list_move_tail(&folio->lru, &unmap_folios);
 				list_add_tail(&dst->lru, &dst_folios);
 				break;
-- 
2.50.1