quoted

The trigger is memblock_reserve() for the complete node memory.  And
this is exactly what FA_DUMP does.  Here again the node has memory but
its all reserved so there is no free memory in the node.

Did you mean populated_zone() when you said zone_populated or have I
mistaken? populated_zone() does return 1 since it checks for
zone->present_pages.

Yes, I meant populated_zone(). Using present pages may have hidden
a long-lived corner case as it was unexpected that an entire node
would be reserved. The old code happened to survive *probably* because
pgdat_reclaimable would look false and kswapd checks for pgdat being
balanced would happen to do the right thing in this case.

Can you check if something like this works?

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index d572b78b65e1..cf64a5456cf6 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h

@@ -830,7 +830,7 @@ unsigned long __init node_memmap_size_bytes(int, unsigned long, unsigned long);

 static inline int populated_zone(struct zone *zone)
 {
-	return (!!zone->present_pages);
+	return (!!zone->managed_pages);
 }

 extern int movable_zone;