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

Davidlohr Bueso
2025-03-14T01:09:01Z

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]);
>+ }
>+}