mm/munlock: delete smp_mb() from __pagevec_lru_add_fn()

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>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

diff --git a/mm/swap.c b/mm/swap.c
index 682a03301a2c05e9e6537106b7f72b744a724bd8..3f770b1ea2c12db2ca508b15b910496fa113473e 100644 (file)
--- 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);