Re: [PATCH v4] mm/migrate: split source folio if it is on deferred split list

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 19, 2024 at 09:45:11PM -0400, Zi Yan wrote:
> +++ b/mm/migrate.c
> @@ -1654,25 +1654,65 @@ static int migrate_pages_batch(struct list_head *from,
>  
>  			/*
>  			 * Large folio migration might be unsupported or
> -			 * the allocation might be failed so we should retry
> -			 * on the same folio with the large folio split
> +			 * the folio is on deferred split list so we should
> +			 * retry on the same folio with the large folio split
>  			 * to normal folios.
>  			 *
>  			 * Split folios are put in split_folios, and
>  			 * we will migrate them after the rest of the
>  			 * list is processed.
>  			 */
> -			if (!thp_migration_supported() && is_thp) {
> -				nr_failed++;
> -				stats->nr_thp_failed++;
> -				if (!try_split_folio(folio, split_folios)) {
> -					stats->nr_thp_split++;
> -					stats->nr_split++;
> +			if (is_thp) {
> +				bool is_on_deferred_list = false;
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +				/*
> +				 * Check without taking split_queue_lock to
> +				 * reduce locking overheads. The worst case is
> +				 * that if the folio is put on the deferred
> +				 * split list after the check, it will be
> +				 * migrated and not put back on the list.
> +				 * The migrated folio will not be split
> +				 * via shrinker during memory pressure.
> +				 */
> +				if (!data_race(list_empty(&folio->_deferred_list))) {
> +					struct deferred_split *ds_queue;
> +					unsigned long flags;
> +
> +					ds_queue =
> +						get_deferred_split_queue(folio);
> +					spin_lock_irqsave(&ds_queue->split_queue_lock,
> +							  flags);
> +					/*
> +					 * Only check if the folio is on
> +					 * deferred split list without removing
> +					 * it. Since the folio can be on
> +					 * deferred_split_scan() local list and
> +					 * removing it can cause the local list
> +					 * corruption. Folio split process
> +					 * below can handle it with the help of
> +					 * folio_ref_freeze().
> +					 */
> +					is_on_deferred_list =
> +						!list_empty(&folio->_deferred_list);
> +					spin_unlock_irqrestore(&ds_queue->split_queue_lock,
> +							       flags);
> +				}
> +#endif
> +				if (!thp_migration_supported() ||
> +						is_on_deferred_list) {
> +					nr_failed++;
> +					stats->nr_thp_failed++;
> +					if (!try_split_folio(folio,
> +							     split_folios)) {
> +						stats->nr_thp_split++;
> +						stats->nr_split++;
> +						continue;
> +					}
> +					stats->nr_failed_pages += nr_pages;
> +					list_move_tail(&folio->lru, ret_folios);
>  					continue;
>  				}
> -				stats->nr_failed_pages += nr_pages;
> -				list_move_tail(&folio->lru, ret_folios);
> -				continue;
>  			}

I don't think we need to try quite this hard.  I don't think we need
to take the lock to be certain if it's on the deferred list -- is
there anything preventing the folio being added to the deferred list
after we drop the lock?

I also don't think we should account this as a thp split since those
are treated by callers as failures.  So maybe this?

+++ b/mm/migrate.c
@@ -1652,6 +1652,17 @@ static int migrate_pages_batch(struct list_head *from,

                        cond_resched();

+                       /*
+                        * The rare folio on the deferred split list should
+                        * be split now.  It should not count as a failure.
+                        */
+                       if (nr_pages > 2 &&
+                           !list_empty(&folio->_deferred_list)) {
+                               if (try_split_folio(folio, from) == 0) {
+                                       is_large = is_thp = false;
+                                       nr_pages = 1;
+                               }
+                       }
                        /*
                         * Large folio migration might be unsupported or
                         * the allocation might be failed so we should retry