Re: [PATCH v2 3/4] mm: thp: use folio_batch to handle THP splitting in deferred_split_scan()

Zi Yan
2025-09-23T15:31:35Z

On 23 Sep 2025, at 5:16, Qi Zheng wrote:

> From: Muchun Song <songmuchun@bytedance.com>
>
> The maintenance of the folio->_deferred_list is intricate because it's
> reused in a local list.
>
> Here are some peculiarities:
>
> 1) When a folio is removed from its split queue and added to a local
> on-stack list in deferred_split_scan(), the ->split_queue_len isn't
> updated, leading to an inconsistency between it and the actual
> number of folios in the split queue.
>
> 2) When the folio is split via split_folio() later, it's removed from
> the local list while holding the split queue lock. At this time,
> this lock protects the local list, not the split queue.
>
> 3) To handle the race condition with a third-party freeing or migrating
> the preceding folio, we must ensure there's always one safe (with
> raised refcount) folio before by delaying its folio_put(). More
> details can be found in commit e66f3185fa04 ("mm/thp: fix deferred
> split queue not partially_mapped"). It's rather tricky.
>
> We can use the folio_batch infrastructure to handle this clearly. In this

Can you add more details on how folio_batch handles the above three concerns
in the original code? That would guide me what to look for during code review.

> case, ->split_queue_len will be consistent with the real number of folios
> in the split queue. If list_empty(&folio->_deferred_list) returns false,
> it's clear the folio must be in its split queue (not in a local list
> anymore).
>
> In the future, we will reparent LRU folios during memcg offline to
> eliminate dying memory cgroups, which requires reparenting the split queue
> to its parent first. So this patch prepares for using
> folio_split_queue_lock_irqsave() as the memcg may change then.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
> mm/huge_memory.c | 84 ++++++++++++++++++++++--------------------------
> 1 file changed, 38 insertions(+), 46 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2f41b8f0d4871..48b51e6230a67 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3781,21 +3781,22 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> struct lruvec *lruvec;
> int expected_refs;
>
> - if (folio_order(folio) > 1 &&
> - !list_empty(&folio->_deferred_list)) {
> - ds_queue->split_queue_len--;
> + if (folio_order(folio) > 1) {
> + if (!list_empty(&folio->_deferred_list)) {
> + ds_queue->split_queue_len--;
> + /*
> + * Reinitialize page_deferred_list after removing the
> + * page from the split_queue, otherwise a subsequent
> + * split will see list corruption when checking the
> + * page_deferred_list.
> + */
> + list_del_init(&folio->_deferred_list);
> + }
> if (folio_test_partially_mapped(folio)) {
> folio_clear_partially_mapped(folio);
> mod_mthp_stat(folio_order(folio),
> MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> }

folio_test_partially_mapped() is done regardless the folio is on _deferred_list
or not, is it because the folio can be on a folio batch and its _deferred_list
is empty?

> - /*
> - * Reinitialize page_deferred_list after removing the
> - * page from the split_queue, otherwise a subsequent
> - * split will see list corruption when checking the
> - * page_deferred_list.
> - */
> - list_del_init(&folio->_deferred_list);
> }
> split_queue_unlock(ds_queue);
> if (mapping) {
> @@ -4194,40 +4195,44 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> struct pglist_data *pgdata = NODE_DATA(sc->nid);
> struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
> unsigned long flags;
> - LIST_HEAD(list);
> - struct folio *folio, *next, *prev = NULL;
> - int split = 0, removed = 0;
> + struct folio *folio, *next;
> + int split = 0, i;
> + struct folio_batch fbatch;
>
> #ifdef CONFIG_MEMCG
> if (sc->memcg)
> ds_queue = &sc->memcg->deferred_split_queue;
> #endif
>
> + folio_batch_init(&fbatch);
> +retry:
> spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> /* Take pin on all head pages to avoid freeing them under us */
> list_for_each_entry_safe(folio, next, &ds_queue->split_queue,
> _deferred_list) {
> if (folio_try_get(folio)) {
> - list_move(&folio->_deferred_list, &list);
> - } else {
> + folio_batch_add(&fbatch, folio);
> + } else if (folio_test_partially_mapped(folio)) {
> /* We lost race with folio_put() */
> - if (folio_test_partially_mapped(folio)) {
> - folio_clear_partially_mapped(folio);
> - mod_mthp_stat(folio_order(folio),
> - MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> - }
> - list_del_init(&folio->_deferred_list);
> - ds_queue->split_queue_len--;
> + folio_clear_partially_mapped(folio);
> + mod_mthp_stat(folio_order(folio),
> + MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> }
> + list_del_init(&folio->_deferred_list);
> + ds_queue->split_queue_len--;

At this point, the folio can be following conditions:
1. deferred_split_scan() gets it,
2. it is freed by folio_put().

In both cases, it is removed from deferred_split_queue, right?

> if (!--sc->nr_to_scan)
> break;
> + if (!folio_batch_space(&fbatch))
> + break;
> }
> spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>
> - list_for_each_entry_safe(folio, next, &list, _deferred_list) {
> + for (i = 0; i < folio_batch_count(&fbatch); i++) {
> bool did_split = false;
> bool underused = false;
> + struct deferred_split *fqueue;
>
> + folio = fbatch.folios[i];
> if (!folio_test_partially_mapped(folio)) {
> /*
> * See try_to_map_unused_to_zeropage(): we cannot
> @@ -4250,38 +4255,25 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> }
> folio_unlock(folio);
> next:
> + if (did_split || !folio_test_partially_mapped(folio))
> + continue;
> /*
> - * split_folio() removes folio from list on success.
> * Only add back to the queue if folio is partially mapped.
> * If thp_underused returns false, or if split_folio fails
> * in the case it was underused, then consider it used and
> * don't add it back to split_queue.
> */
> - if (did_split) {
> - ; /* folio already removed from list */
> - } else if (!folio_test_partially_mapped(folio)) {
> - list_del_init(&folio->_deferred_list);
> - removed++;
> - } else {
> - /*
> - * That unlocked list_del_init() above would be unsafe,
> - * unless its folio is separated from any earlier folios
> - * left on the list (which may be concurrently unqueued)
> - * by one safe folio with refcount still raised.
> - */
> - swap(folio, prev);
> + fqueue = folio_split_queue_lock_irqsave(folio, &flags);
> + if (list_empty(&folio->_deferred_list)) {
> + list_add_tail(&folio->_deferred_list, &fqueue->split_queue);
> + fqueue->split_queue_len++;

fqueue should be the same as ds_queue, right? Just want to make sure
I understand the code.

> }
> - if (folio)
> - folio_put(folio);
> + split_queue_unlock_irqrestore(fqueue, flags);
> }
> + folios_put(&fbatch);
>
> - spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> - list_splice_tail(&list, &ds_queue->split_queue);
> - ds_queue->split_queue_len -= removed;
> - spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> -
> - if (prev)
> - folio_put(prev);
> + if (sc->nr_to_scan)
> + goto retry;
>
> /*
> * Stop shrinker if we didn't split any page, but the queue is empty.
> --
> 2.20.1

Best Regards,
Yan, Zi