vmscan: prevent background aging of anon page in no swap system

Ying Han reported that backing aging of anon pages in no swap system
causes unnecessary TLB flush.

When I sent a patch(69c85481), I wanted this patch but Rik pointed out
and allowed aging of anon pages to give a chance to promote from inactive
to active LRU.

It has a two problem.

1) non-swap system

Never make sense to age anon pages.

2) swap configured but still doesn't swapon

It doesn't make sense to age anon pages until swap-on time.  But it's
arguable.  If we have aged anon pages by swapon, VM have moved anon pages
from active to inactive.  And in the time swapon by admin, the VM can't
reclaim hot pages so we can protect hot pages swapout.

But let's think about it.  When does swap-on happen?  It depends on admin.
 we can't expect it.  Nonetheless, we have done aging of anon pages to
protect hot pages swapout.  It means we lost run time overhead when below
high watermark but gain hot page swap-[in/out] overhead when VM decide
swapout.  Is it true?  Let's think more detail.  We don't promote anon
pages in case of non-swap system.  So even though VM does aging of anon
pages, the pages would be in inactive LRU for a long time.  It means many
of pages in there would mark access bit again.  So access bit hot/code
separation would be pointless.

This patch prevents unnecessary anon pages demotion in not-yet-swapon and
non-configured swap system.  Even, in non-configuared swap system
inactive_anon_is_low can be compiled out.

It could make side effect that hot anon pages could swap out when admin
does swap on.  But I think sooner or later it would be steady state.  So
it's not a big problem.

We could lose someting but gain more thing(TLB flush and unnecessary
function call to demote anon pages).
Signed-off-by: Ying Han <yinghan@google.com>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1505,6 +1505,7 @@ static void shrink_active_list(unsigned long nr_pages, struct zone *zone,
 	spin_unlock_irq(&zone->lru_lock);
 }
 
+#ifdef CONFIG_SWAP
 static int inactive_anon_is_low_global(struct zone *zone)
 {
 	unsigned long active, inactive;
@@ -1530,12 +1531,26 @@ static int inactive_anon_is_low(struct zone *zone, struct scan_control *sc)
 {
 	int low;
 
+	/*
+	 * If we don't have swap space, anonymous page deactivation
+	 * is pointless.
+	 */
+	if (!total_swap_pages)
+		return 0;
+
 	if (scanning_global_lru(sc))
 		low = inactive_anon_is_low_global(zone);
 	else
 		low = mem_cgroup_inactive_anon_is_low(sc->mem_cgroup);
 	return low;
 }
+#else
+static inline int inactive_anon_is_low(struct zone *zone,
+					struct scan_control *sc)
+{
+	return 0;
+}
+#endif
 
 static int inactive_file_is_low_global(struct zone *zone)
 {
@@ -1781,7 +1796,7 @@ static void shrink_zone(int priority, struct zone *zone,
 	 * Even if we did not try to evict anon pages at all, we want to
 	 * rebalance the anon lru active/inactive ratio.
 	 */
-	if (inactive_anon_is_low(zone, sc) && nr_swap_pages > 0)
+	if (inactive_anon_is_low(zone, sc))
 		shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
 
 	throttle_vm_writeout(sc->gfp_mask);