Re: [RFC PATCH 2/4] mm: kpromoted: Hot page info collection and promotion daemon

[Davidlohr Bueso]

On Thu, 06 Mar 2025, Bharata B Rao wrote:

> +/*
> + * Go thro' page hotness information and migrate pages if required.
> + *
> + * Promoted pages are not longer tracked in the hot list.
> + * Cold pages are pruned from the list as well.
> + *
> + * TODO: Batching could be done
> + */
> +static void kpromoted_migrate(pg_data_t *pgdat)
> +{
> +	int nid = pgdat->node_id;
> +	struct page_hotness_info *phi;
> +	struct hlist_node *tmp;
> +	int nr_bkts = HASH_SIZE(page_hotness_hash);
> +	int bkt;
> +
> +	for (bkt = 0; bkt < nr_bkts; bkt++) {
> +		mutex_lock(&page_hotness_lock[bkt]);
> +		hlist_for_each_entry_safe(phi, tmp, &page_hotness_hash[bkt], hnode) {
> +			if (phi->hot_node != nid)
> +				continue;
> +
> +			if (page_should_be_promoted(phi)) {
> +				count_vm_event(KPROMOTED_MIG_CANDIDATE);
> +				if (!kpromote_page(phi)) {
> +					count_vm_event(KPROMOTED_MIG_PROMOTED);
> +					hlist_del_init(&phi->hnode);
> +					kfree(phi);
> +				}
> +			} else {
> +				/*
> +				 * Not a suitable page or cold page, stop tracking it.
> +				 * TODO: Identify cold pages and drive demotion?
> +				 */

I don't think kpromoted should drive demotion at all. No one is complaining about migrate
in lieu of discard, and there is also proactive reclaim which users can trigger. All the
in-kernel problems are wrt promotion. The simpler any of these kthreads are the better.

> +				count_vm_event(KPROMOTED_MIG_DROPPED);
> +				hlist_del_init(&phi->hnode);
> +				kfree(phi);
> +			}
> +		}
> +		mutex_unlock(&page_hotness_lock[bkt]);
> +	}
> +}