[PATCH] mm: vmscan: reduce throttling due to a failure to make progress -fix

[Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>]

Hugh Dickins reported the following

	My tmpfs swapping load (tweaked to use huge pages more heavily
	than in real life) is far from being a realistic load: but it was
	notably slowed down by your throttling mods in 5.16-rc, and this
	patch makes it well again - thanks.

	But: it very quickly hit NULL pointer until I changed that last
	line to

        if (first_pgdat)
                consider_reclaim_throttle(first_pgdat, sc);

The likely issue is that huge pages are a major component of the test
workload. When this is the case, first_pgdat may never get set if
compaction is ready to continue due to this check

        if (IS_ENABLED(CONFIG_COMPACTION) &&
            sc->order > PAGE_ALLOC_COSTLY_ORDER &&
            compaction_ready(zone, sc)) {
                sc->compaction_ready = true;
                continue;
        }

If this was true for every zone in the zonelist, first_pgdat would never
get set resulting in a NULL pointer exception.

This is a fix to the mmotm patch
mm-vmscan-reduce-throttling-due-to-a-failure-to-make-progress.patch

Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 mm/vmscan.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4c4d5f6cd8a3..700434db5735 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3530,7 +3530,8 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
 		shrink_node(zone->zone_pgdat, sc);
 	}
 
-	consider_reclaim_throttle(first_pgdat, sc);
+	if (first_pgdat)
+		consider_reclaim_throttle(first_pgdat, sc);
 
 	/*
 	 * Restore to original mask to avoid the impact on the caller if we