[PATCH 4/6] mm, kswapd: wake up kcompactd when kswapd had too many failures

[Vlastimil Babka <vbabka@suse.cz>]

This patch deals with a corner case found when testing kcompactd with a very
simple testcase that first fragments memory (by creating a large shmem file and
then punching hole in every even page) and then uses artificial order-9
GFP_NOWAIT allocations in a loop. This is freshly after virtme-run boot in KVM
and no other activity.

What happens is that after few kswapd runs, there are no more reclaimable
pages, and high-order pages can only be created by compaction. Because kswapd
can't reclaim anything, pgdat->kswapd_failures increases up to
MAX_RECLAIM_RETRIES and kswapd is no longer woken up. Thus kcompactd is also
not woken up. After this patch, we will try to wake up kcompactd immediately
instead of kswapd.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/vmscan.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a3f914c88dea..18ad0cd0c0f5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3578,9 +3578,15 @@ 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)
+	/*
+	 * Hopeless node, leave it to direct reclaim. For high-order
+	 * allocations, try to wake up kcompactd instead.
+	 */
+	if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES) {
+		if (order)
+			wakeup_kcompactd(pgdat, order, classzone_idx);
 		return;
+	}
 
 	if (pgdat_balanced(pgdat, order, classzone_idx))
 		return;
-- 
2.13.3

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