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

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.

Link: https://lore.kernel.org/20261019184850.3427905-1-gourry@gourry.net
Fixes: e7a5f249e6 ("mm: re-enable kswapd when memory pressure subsides or demotion is toggled")
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Chanwon Park <flyinrm@gmail.com>
Cc: <stable@vger.kernel.org>

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;