mm: munlock: remove redundant get_page/put_page pair on the fast path

The performance of the fast path in munlock_vma_range() can be further
improved by avoiding atomic ops of a redundant get_page()/put_page() pair.

When calling get_page() during page isolation, we already have the pin
from follow_page_mask().  This pin will be then returned by
__pagevec_lru_add(), after which we do not reference the pages anymore.

After this patch, an 8% speedup was measured for munlocking a 56GB large
memory area with THP disabled.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Jörn Engel <joern@logfs.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Michel Lespinasse <walken@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mlock.c

@@ -303,8 +303,10 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
 			if (PageLRU(page)) {
 				lruvec = mem_cgroup_page_lruvec(page, zone);
 				lru = page_lru(page);
-
-				get_page(page);
+				/*
+				 * We already have pin from follow_page_mask()
+				 * so we can spare the get_page() here.
+				 */
 				ClearPageLRU(page);
 				del_page_from_lru_list(page, lruvec, lru);
 			} else {
@@ -336,25 +338,25 @@ skip_munlock:
 			lock_page(page);
 			if (!__putback_lru_fast_prepare(page, &pvec_putback,
 					&pgrescued)) {
-				/* Slow path */
+				/*
+				 * Slow path. We don't want to lose the last
+				 * pin before unlock_page()
+				 */
+				get_page(page); /* for putback_lru_page() */
 				__munlock_isolated_page(page);
 				unlock_page(page);
+				put_page(page); /* from follow_page_mask() */
 			}
 		}
 	}
 
-	/* Phase 3: page putback for pages that qualified for the fast path */
+	/*
+	 * Phase 3: page putback for pages that qualified for the fast path
+	 * This will also call put_page() to return pin from follow_page_mask()
+	 */
 	if (pagevec_count(&pvec_putback))
 		__putback_lru_fast(&pvec_putback, pgrescued);
 
-	/* Phase 4: put_page to return pin from follow_page_mask() */
-	for (i = 0; i < nr; i++) {
-		struct page *page = pvec->pages[i];
-
-		if (page)
-			put_page(page);
-	}
-
 	pagevec_reinit(pvec);
 }