[PATCH 09/13] mm/munlock: delete smp_mb() from __pagevec_lru_add_fn()

Hugh Dickins
2022-02-06T21:45:58Z

My reading of comment on smp_mb__after_atomic() in __pagevec_lru_add_fn()
says that it can now be deleted; and that remains so when the next patch
is added.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/swap.c | 37 +++++++++----------------------------
1 file changed, 9 insertions(+), 28 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index 682a03301a2c..3f770b1ea2c1 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -1025,37 +1025,18 @@ static void __pagevec_lru_add_fn(struct folio *folio, struct lruvec *lruvec)

VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);

+ folio_set_lru(folio);
/*
- * A folio becomes evictable in two ways:
- * 1) Within LRU lock [munlock_vma_page() and __munlock_pagevec()].
- * 2) Before acquiring LRU lock to put the folio on the correct LRU
- * and then
- * a) do PageLRU check with lock [check_move_unevictable_pages]
- * b) do PageLRU check before lock [clear_page_mlock]
- *
- * (1) & (2a) are ok as LRU lock will serialize them. For (2b), we need
- * following strict ordering:
- *
- * #0: __pagevec_lru_add_fn #1: clear_page_mlock
- *
- * folio_set_lru() folio_test_clear_mlocked()
- * smp_mb() // explicit ordering // above provides strict
- * // ordering
- * folio_test_mlocked() folio_test_lru()
+ * Is an smp_mb__after_atomic() still required here, before
+ * folio_evictable() tests PageMlocked, to rule out the possibility
+ * of stranding an evictable folio on an unevictable LRU? I think
+ * not, because munlock_page() only clears PageMlocked while the LRU
+ * lock is held.
*
- *
- * if '#1' does not observe setting of PG_lru by '#0' and
- * fails isolation, the explicit barrier will make sure that
- * folio_evictable check will put the folio on the correct
- * LRU. Without smp_mb(), folio_set_lru() can be reordered
- * after folio_test_mlocked() check and can make '#1' fail the
- * isolation of the folio whose mlocked bit is cleared (#0 is
- * also looking at the same folio) and the evictable folio will
- * be stranded on an unevictable LRU.
+ * (That is not true of __page_cache_release(), and not necessarily
+ * true of release_pages(): but those only clear PageMlocked after
+ * put_page_testzero() has excluded any other users of the page.)
*/
- folio_set_lru(folio);
- smp_mb__after_atomic();
-
if (folio_evictable(folio)) {
if (was_unevictable)
__count_vm_events(UNEVICTABLE_PGRESCUED, nr_pages);
--
2.34.1