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

Zi Yan
2025-09-24T14:57:15Z

On 24 Sep 2025, at 5:57, Qi Zheng wrote:

> Hi Zi,
>
> On 9/23/25 11:31 PM, Zi Yan wrote:
>> 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.
>
> Sure.
>
> For 1), after adding folio to folio_batch, we immediatelly decrement the
> ds_queue->split_queue_len, so there are no more inconsistencies.
>
> For 2), after adding folio to folio_batch, we will see list_empty() in
> __folio_split(), so there is no longer a situation where
> split_queue_lock protects the local list.
>
> For 3), after adding folio to folio_batch, we call folios_put() at the
> end to decrement the refcount of folios, which looks more natural.
>
>>
>>> 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?
>
> Yes.
>
>>
>>> - /*
>>> - * 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?
>
> Right. For the case 1), we may add folio back to deferred_split_queue.
>
>>
>>> 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.
>
> After patch #4, fqueue may be the deferred_split of parent memcg.
Thank you for the explanation. The changes look good to me.

Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi