[PATCH v2 11/26] mm/fbatch: remove migration's PAGE_WAS_MLOCKED lru_add_drain()

[Hugh Dickins <hughd@google.com>]

A welcome side-effect of mm/mlock.c's mod_mlock_count() succeeding on
folios on the per-cpu lru_add fbatch, is that migrate_folio_move() no
longer has to lru_add_drain() before remove_migration_ptes() restores
a PAGE_WAS_MLOCKED mlock_count: so remove PAGE_WAS_MLOCKED altogether.

(Re the "We would like to do something similar for the old page, when
unsuccessful" comment above it: that may be easier now, but involve
some rearrangement: not researched, so just leave the comment as is.)

However: it was uncommon before, for isolate_migratepages_block() to
meet a folio on LRU, with suitable refcount, marked mlocked but not yet
unevictable: so that got missed when sysctl_compact_unevictable_allowed
should not allow it. But now it's a more common case, so check if the
folio will become unevictable, by using !folio_evictable() instead.

That is more expensive than testing mlocked and unevictable bits: but
it is consistent with the decision made once the fbatch gets drained,
and is_inaccessible further down depends on the right is_unevictable.
Optimize those checks better if they show a regression in practice.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/compaction.c | 2 +-
 mm/migrate.c    | 8 +-------
 2 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 9e045a90ba21..1290b9170cb2 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1113,7 +1113,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		if (!folio_test_lru(folio))
 			goto isolate_fail_put;
 
-		is_unevictable = folio_test_unevictable(folio);
+		is_unevictable = !folio_evictable(folio);
 
 		/* Compaction might skip unevictable pages but CMA takes them */
 		if (!(mode & ISOLATE_UNEVICTABLE) && is_unevictable)
diff --git a/mm/migrate.c b/mm/migrate.c
index 1939d6ed19c9..364015cbef33 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1147,8 +1147,7 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
  */
 enum {
 	FOLIO_WAS_MAPPED = BIT(0),
-	FOLIO_WAS_MLOCKED = BIT(1),
-	FOLIO_OLD_STATES = FOLIO_WAS_MAPPED | FOLIO_WAS_MLOCKED,
+	FOLIO_OLD_STATES = FOLIO_WAS_MAPPED,
 };
 
 static void __migrate_folio_record(struct folio *dst,
@@ -1258,8 +1257,6 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 		folio_lock(src);
 	}
 	locked = true;
-	if (folio_test_mlocked(src))
-		old_folio_state |= FOLIO_WAS_MLOCKED;
 
 	if (folio_test_writeback(src)) {
 		/*
@@ -1410,9 +1407,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 	 * isolated from the unevictable LRU: but this case is the easiest.
 	 */
 	folio_add_lru(dst);
-	if (old_folio_state & FOLIO_WAS_MLOCKED)
-		lru_add_drain();
-
 	if (old_folio_state & FOLIO_WAS_MAPPED)
 		remove_migration_ptes(src, dst, 0);
 
-- 
2.51.0