[PATCH 2/2] mm: swap: use smp_mb__after_atomic() to order LRU bit set

[Yang Shi]

Memory barrier is needed after setting LRU bit, but smp_mb() is too
strong.  Some architectures, i.e. x86, imply memory barrier with atomic
operations, so replacing it with smp_mb__after_atomic() sounds better,
which is nop on strong ordered machines, and full memory barriers on
others.  With this change the vm-calability cases would perform better
on x86, I saw total 6% improvement with this patch and previous inline
fix.

The test data (lru-file-readtwice throughput) against v5.6-rc4:
	mainline	w/ inline fix	w/ both (adding this)
	150MB		154MB		159MB

Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs")
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
 mm/swap.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index cf39d24..118bac4 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -945,20 +945,20 @@ static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
 	 * #0: __pagevec_lru_add_fn		#1: clear_page_mlock
 	 *
 	 * SetPageLRU()				TestClearPageMlocked()
-	 * smp_mb() // explicit ordering	// above provides strict
+	 * MB() 	// explicit ordering	// above provides strict
 	 *					// ordering
 	 * PageMlocked()			PageLRU()
 	 *
 	 *
 	 * if '#1' does not observe setting of PG_lru by '#0' and fails
 	 * isolation, the explicit barrier will make sure that page_evictable
-	 * check will put the page in correct LRU. Without smp_mb(), SetPageLRU
+	 * check will put the page in correct LRU. Without MB(), SetPageLRU
 	 * can be reordered after PageMlocked check and can make '#1' to fail
 	 * the isolation of the page whose Mlocked bit is cleared (#0 is also
 	 * looking at the same page) and the evictable page will be stranded
 	 * in an unevictable LRU.
 	 */
-	smp_mb();
+	smp_mb__after_atomic();
 
 	if (page_evictable(page)) {
 		lru = page_lru(page);
-- 
1.8.3.1