Re: [PATCH v9 2/2] mm/vmscan: select the closest perferred node in demote_folio_list()

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Jan 14, 2026 at 08:53:03PM +0000, Bing Jiao wrote:
> The preferred demotion node (migration_target_control.nid) should be the
> one closest to the source node to minimize migration latency.  Currently,
> a discrepancy exists where demote_folio_list() randomly selects an allowed
> node if the preferred node from next_demotion_node() is not set in
> mems_effective.
> 
> To address it, update next_demotion_node() to select a preferred target
> against allowed nodes; and to return the closest demotion target if all
> preferred nodes are not in mems_effective via next_demotion_node().
> 
> It ensures that the preferred demotion target is consistently the closest
> available node to the source node.
> 
> Signed-off-by: Bing Jiao <bingjiao@google.com>

One nit below:

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

[...]

> @@ -320,16 +320,17 @@ void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets)
>  /**
>   * next_demotion_node() - Get the next node in the demotion path
>   * @node: The starting node to lookup the next node
> + * @allowed_mask: The pointer to allowed node mask
>   *
>   * Return: node id for next memory node in the demotion path hierarchy
>   * from @node; NUMA_NO_NODE if @node is terminal.  This does not keep
>   * @node online or guarantee that it *continues* to be the next demotion
>   * target.
>   */
> -int next_demotion_node(int node)
> +int next_demotion_node(int node, const nodemask_t *allowed_mask)
>  {
>  	struct demotion_nodes *nd;
> -	int target;
> +	nodemask_t mask;
> 
>  	if (!node_demotion)
>  		return NUMA_NO_NODE;
> @@ -344,6 +345,10 @@ int next_demotion_node(int node)
>  	 * node_demotion[] reads need to be consistent.
>  	 */
>  	rcu_read_lock();
> +	/* Filter out nodes that are not in allowed_mask. */
> +	nodes_and(mask, nd->preferred, *allowed_mask);
> +	rcu_read_unlock();
> +
>  	/*
>  	 * If there are multiple target nodes, just select one
>  	 * target node randomly.
> @@ -356,10 +361,16 @@ int next_demotion_node(int node)
>  	 * caching issue, which seems more complicated. So selecting
>  	 * target node randomly seems better until now.
>  	 */
> -	target = node_random(&nd->preferred);
> -	rcu_read_unlock();
> +	if (!nodes_empty(mask))
> +		return node_random(&mask);
> 
> -	return target;
> +	/*
> +	 * Preferred nodes are not in allowed_mask. Filp bits in

Filp -> Flip

> +	 * allowed_mask as used node mask. Then, use it to get the
> +	 * closest demotion target.
> +	 */
> +	nodes_complement(mask, *allowed_mask);
> +	return find_next_best_node(node, &mask);
>  }
>