[RFC] mem-hotplug: shall we skip unmovable node when doing numa balance?

[Xishi Qiu]

On mem-hotplug system, there is a problem, please see the following case.

memtester xxG, the memory will be alloced on a movable node. And after numa
balancing, the memory may be migrated to the other node, it may be a unmovable
node. This will reduce the free memory of the unmovable node, and may be oom
later.

My question is that shall we skip unmovable node when doing numa balance?
or just let the manager set some numa policies?

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 057964d..f0954ac 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2334,6 +2334,13 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
 out:
 	mpol_cond_put(pol);
 
+	/* Skip unmovable nodes when do numa balancing */
+	if (movable_node_enabled && ret != -1) {
+		zone = NODE_DATA(ret)->node_zones + MAX_NR_ZONES - 1;
+		if (!populated_zone(zone))
+			ret = -1;
+	}
+
 	return ret;
 }

Thanks,
Xishi Qiu