mm, page_alloc: wakeup kcompactd even if kswapd cannot free more memory

Kswapd will not wakeup if per-zone watermarks are not failing or if too
many previous attempts at background reclaim have failed.

This can be true if there is a lot of free memory available.  For high-
order allocations, kswapd is responsible for waking up kcompactd for
background compaction.  If the zone is not below its watermarks or
reclaim has recently failed (lots of free memory, nothing left to
reclaim), kcompactd does not get woken up.

When __GFP_DIRECT_RECLAIM is not allowed, allow kcompactd to still be
woken up even if kswapd will not reclaim.  This allows high-order
allocations, such as thp, to still trigger background compaction even
when the zone has an abundance of free memory.

Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1803111659420.209721@chino.kir.corp.google.com
Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 23 insertions, 9 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 976be140a8ce..4390a8d5be41 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3539,16 +3539,21 @@ kswapd_try_sleep:
 }
 
 /*
- * A zone is low on free memory, so wake its kswapd task to service it.
+ * A zone is low on free memory or too fragmented for high-order memory.  If
+ * kswapd should reclaim (direct reclaim is deferred), wake it up for the zone's
+ * pgdat.  It will wake up kcompactd after reclaiming memory.  If kswapd reclaim
+ * has failed or is not needed, still wake up kcompactd if only compaction is
+ * needed.
  */
-void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
+void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
+		   enum zone_type classzone_idx)
 {
 	pg_data_t *pgdat;
 
 	if (!managed_zone(zone))
 		return;
 
-	if (!cpuset_zone_allowed(zone, GFP_KERNEL | __GFP_HARDWALL))
+	if (!cpuset_zone_allowed(zone, gfp_flags))
 		return;
 	pgdat = zone->zone_pgdat;
 	pgdat->kswapd_classzone_idx = kswapd_classzone_idx(pgdat,
@@ -3557,14 +3562,23 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
 	if (!waitqueue_active(&pgdat->kswapd_wait))
 		return;
 
-	/* Hopeless node, leave it to direct reclaim */
-	if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
-		return;
-
-	if (pgdat_balanced(pgdat, order, classzone_idx))
+	/* Hopeless node, leave it to direct reclaim if possible */
+	if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ||
+	    pgdat_balanced(pgdat, order, classzone_idx)) {
+		/*
+		 * There may be plenty of free memory available, but it's too
+		 * fragmented for high-order allocations.  Wake up kcompactd
+		 * and rely on compaction_suitable() to determine if it's
+		 * needed.  If it fails, it will defer subsequent attempts to
+		 * ratelimit its work.
+		 */
+		if (!(gfp_flags & __GFP_DIRECT_RECLAIM))
+			wakeup_kcompactd(pgdat, order, classzone_idx);
 		return;
+	}
 
-	trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, classzone_idx, order);
+	trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, classzone_idx, order,
+				      gfp_flags);
 	wake_up_interruptible(&pgdat->kswapd_wait);
 }