[PATCH] mm/page_alloc: let the highest memory node recover from hopeless state

[Gregory Price (Meta)]

free_frozen_page_commit() clears a hopeless node's kswapd failures once
its zone recovers above the high watermark, but only if another memory
node exists for allocations to fall back to.

The test `next_memory_node(nid) < MAX_NUMNODES` finds only memory nodes
with a higher id. The highest-numbered memory node therefore never
recovers on this path, and its kswapd stays disabled until reclaim on
that node makes progress and balances it, or demotion is re-enabled.

In many cases, direct reclaim many never fully run against a lower
node (CXL/kmem), and that means its kswapd can end up full disabled
in rare situations.

To reproduce, fill the highest memory node with bound, non-THP anonymous
memory and no swap until kswapd gives up, then free it: /proc/zoneinfo
keeps showing node_unreclaimable 1 for that node. A lower node recovers.

Count the memory nodes instead. Adding first_memory_node != nid to the
existing test would also work, but a count says directly what the check
means: some other memory node exists.

Fixes: e7a5f249e6db ("mm: re-enable kswapd when memory pressure subsides or demotion is toggled")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 417c427b196d9..6d4f03faf96ee 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2929,7 +2929,7 @@ static bool free_frozen_page_commit(struct zone *zone,
 		 * kswapd work again by resetting kswapd_failures.
 		 */
 		if (kswapd_test_hopeless(pgdat) &&
-		    next_memory_node(pgdat->node_id) < MAX_NUMNODES)
+		    num_node_state(N_MEMORY) > 1)
 			kswapd_clear_hopeless(pgdat, KSWAPD_CLEAR_HOPELESS_PCP);
 	}
 	return ret;
-- 
2.53.0-Meta