[patch] mm: move pagevec stripping to save unlock-relock

[Johannes Weiner <hannes@cmpxchg.org>]

In shrink_active_list() after the deactivation loop, we strip buffer
heads from the potentially remaining pages in the pagevec.

Currently, this drops the zone's lru lock for stripping, only to
reacquire it again afterwards to update statistics.

It is not necessary to strip the pages before updating the stats, so
move the whole thing out of the protected region and save the extra
locking.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/vmscan.c |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1298,14 +1298,11 @@ static void shrink_active_list(unsigned 
 	}
 	__mod_zone_page_state(zone, NR_LRU_BASE + lru, pgmoved);
 	pgdeactivate += pgmoved;
-	if (buffer_heads_over_limit) {
-		spin_unlock_irq(&zone->lru_lock);
-		pagevec_strip(&pvec);
-		spin_lock_irq(&zone->lru_lock);
-	}
 	__count_zone_vm_events(PGREFILL, zone, pgscanned);
 	__count_vm_events(PGDEACTIVATE, pgdeactivate);
 	spin_unlock_irq(&zone->lru_lock);
+	if (buffer_heads_over_limit)
+		pagevec_strip(&pvec);
 	if (vm_swap_full())
 		pagevec_swap_free(&pvec);
 

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