On Wed, Jun 22, 2016 at 04:42:06PM +0800, Hillf Danton wrote:

quoted

 	/*
-	 * If a zone reaches its high watermark, consider it to be no longer
-	 * congested. It's possible there are dirty pages backed by congested
-	 * BDIs but as pressure is relieved, speculatively avoid congestion
-	 * waits.
+	 * Fragmentation may mean that the system cannot be rebalanced for
+	 * high-order allocations. If twice the allocation size has been
+	 * reclaimed then recheck watermarks only at order-0 to prevent
+	 * excessive reclaim. Assume that a process requested a high-order
+	 * can direct reclaim/compact.
 	 */
-	if (pgdat_reclaimable(zone->zone_pgdat) &&
-	    zone_balanced(zone, sc->order, false, 0, classzone_idx)) {
-		clear_bit(PGDAT_CONGESTED, &pgdat->flags);
-		clear_bit(PGDAT_DIRTY, &pgdat->flags);
-	}
+	if (sc->order && sc->nr_reclaimed >= 2UL << sc->order)
+		sc->order = 0;

Reclaim order is changed here.
Btw, I find no such change in current code.

It is reintroducing a check removed by commit accf62422b3a ("mm, kswapd: replace
kswapd compaction with waking up kcompactd"). That patch had kswapd
always check at order-0 once kswapd is awake in pgdat_balanced but would
still take at least one pass through reclaiming so kcompactd potentially
makes progress.

This patch removes pgdat_balanced entirely and zone_balanced() checks the
order it is asked like it used to. Hence, it is necessary to reset sc->order
once progress is made or kswapd potentially stays awake reclaiming pages
until a high-order page is freed.

-- 
Mel Gorman
SUSE Labs

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