[RFC PATCH v4 2/6] mm/migrate: skip data copy for already-copied folios

[Shivank Garg]

Add a PAGE_ALREADY_COPIED flag to the dst->private migration state.
When set, __migrate_folio() skips folio_mc_copy() and performs
metadata-only migration. All callers currently pass
already_copied=false. The batch-copy path enables it in a later patch.

Move the dst->private state enum earlier in the file so
__migrate_folio() and move_to_new_folio() can see PAGE_ALREADY_COPIED.

Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
---
 mm/migrate.c | 52 +++++++++++++++++++++++++++++++---------------------
 1 file changed, 31 insertions(+), 21 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 1bf2cf8c44dd..1d8c1fb627c9 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -848,6 +848,18 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
 }
 EXPORT_SYMBOL(folio_migrate_flags);
 
+/*
+ * To record some information during migration, we use unused private
+ * field of struct folio of the newly allocated destination folio.
+ * This is safe because nobody is using it except us.
+ */
+enum {
+	PAGE_WAS_MAPPED = BIT(0),
+	PAGE_WAS_MLOCKED = BIT(1),
+	PAGE_ALREADY_COPIED = BIT(2),
+	PAGE_OLD_STATES = PAGE_WAS_MAPPED | PAGE_WAS_MLOCKED | PAGE_ALREADY_COPIED,
+};
+
 /************************************************************
  *                    Migration functions
  ***********************************************************/
@@ -857,14 +869,20 @@ static int __migrate_folio(struct address_space *mapping, struct folio *dst,
 			   enum migrate_mode mode)
 {
 	int rc, expected_count = folio_expected_ref_count(src) + 1;
+	bool already_copied = ((unsigned long)dst->private & PAGE_ALREADY_COPIED);
+
+	if (already_copied)
+		dst->private = NULL;
 
 	/* Check whether src does not have extra refs before we do more work */
 	if (folio_ref_count(src) != expected_count)
 		return -EAGAIN;
 
-	rc = folio_mc_copy(dst, src);
-	if (unlikely(rc))
-		return rc;
+	if (!already_copied) {
+		rc = folio_mc_copy(dst, src);
+		if (unlikely(rc))
+			return rc;
+	}
 
 	rc = __folio_migrate_mapping(mapping, dst, src, expected_count);
 	if (rc)
@@ -1088,7 +1106,7 @@ static int fallback_migrate_folio(struct address_space *mapping,
  *     0 - success
  */
 static int move_to_new_folio(struct folio *dst, struct folio *src,
-				enum migrate_mode mode)
+		enum migrate_mode mode, bool already_copied)
 {
 	struct address_space *mapping = folio_mapping(src);
 	int rc = -EAGAIN;
@@ -1096,6 +1114,9 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
 	VM_BUG_ON_FOLIO(!folio_test_locked(src), src);
 	VM_BUG_ON_FOLIO(!folio_test_locked(dst), dst);
 
+	if (already_copied)
+		dst->private = (void *)(unsigned long)PAGE_ALREADY_COPIED;
+
 	if (!mapping)
 		rc = migrate_folio(mapping, dst, src, mode);
 	else if (mapping_inaccessible(mapping))
@@ -1127,17 +1148,6 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
 	return rc;
 }
 
-/*
- * To record some information during migration, we use unused private
- * field of struct folio of the newly allocated destination folio.
- * This is safe because nobody is using it except us.
- */
-enum {
-	PAGE_WAS_MAPPED = BIT(0),
-	PAGE_WAS_MLOCKED = BIT(1),
-	PAGE_OLD_STATES = PAGE_WAS_MAPPED | PAGE_WAS_MLOCKED,
-};
-
 static void __migrate_folio_record(struct folio *dst,
 				   int old_page_state,
 				   struct anon_vma *anon_vma)
@@ -1353,7 +1363,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 			      struct folio *src, struct folio *dst,
 			      enum migrate_mode mode, enum migrate_reason reason,
-			      struct list_head *ret)
+			      struct list_head *ret, bool already_copied)
 {
 	int rc;
 	int old_page_state = 0;
@@ -1371,7 +1381,7 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 		goto out_unlock_both;
 	}
 
-	rc = move_to_new_folio(dst, src, mode);
+	rc = move_to_new_folio(dst, src, mode, already_copied);
 	if (rc)
 		goto out;
 
@@ -1519,7 +1529,7 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
 	}
 
 	if (!folio_mapped(src))
-		rc = move_to_new_folio(dst, src, mode);
+		rc = move_to_new_folio(dst, src, mode, false);
 
 	if (page_was_mapped)
 		remove_migration_ptes(src, !rc ? dst : src, ttu);
@@ -1703,7 +1713,7 @@ static void migrate_folios_move(struct list_head *src_folios,
 		struct list_head *ret_folios,
 		struct migrate_pages_stats *stats,
 		int *retry, int *thp_retry, int *nr_failed,
-		int *nr_retry_pages)
+		int *nr_retry_pages, bool already_copied)
 {
 	struct folio *folio, *folio2, *dst, *dst2;
 	bool is_thp;
@@ -1720,7 +1730,7 @@ static void migrate_folios_move(struct list_head *src_folios,
 
 		rc = migrate_folio_move(put_new_folio, private,
 				folio, dst, mode,
-				reason, ret_folios);
+				reason, ret_folios, already_copied);
 		/*
 		 * The rules are:
 		 *	0: folio will be freed
@@ -1977,7 +1987,7 @@ static int migrate_pages_batch(struct list_head *from,
 		migrate_folios_move(&unmap_folios, &dst_folios,
 				put_new_folio, private, mode, reason,
 				ret_folios, stats, &retry, &thp_retry,
-				&nr_failed, &nr_retry_pages);
+				&nr_failed, &nr_retry_pages, false);
 	}
 	nr_failed += retry;
 	stats->nr_thp_failed += thp_retry;
-- 
2.43.0