Re: [PATCH v4 3/4] mm: thp: use folio_batch to handle THP splitting in deferred_split_scan()

Qi Zheng
2025-10-13T07:28:41Z

Hi Shakeel,

On 10/7/25 7:16 AM, Shakeel Butt wrote:
> On Sat, Oct 04, 2025 at 12:53:17AM +0800, 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.
>
> I think the above text needs some massaging. Rather than saying lock
> protects the local list, I think, it would be better to say that the
> lock is not needed as it is not protecting anything.

Make sense, will do.

>
>>
>> 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
>> 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>
>> Reviewed-by: Zi Yan <ziy@nvidia.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>
> One nit below.
>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

Thanks!

>
>> ---
>> mm/huge_memory.c | 85 ++++++++++++++++++++++--------------------------
>> 1 file changed, 39 insertions(+), 46 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 134666503440d..59ddebc9f3232 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3782,21 +3782,22 @@ 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);
>> }
>> - /*
>> - * 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) {
>> @@ -4185,35 +4186,40 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>> {
>> struct deferred_split *ds_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;
>>
>> + folio_batch_init(&fbatch);
>> +
>> +retry:
>> ds_queue = split_queue_lock_irqsave(sc->nid, sc->memcg, &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--;
>> if (!--sc->nr_to_scan)
>> break;
>> + if (!folio_batch_space(&fbatch))
>> + break;
>> }
>> split_queue_unlock_irqrestore(ds_queue, 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
>> @@ -4236,38 +4242,25 @@ 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++;
>> }
>> - if (folio)
>> - folio_put(folio);
>> + split_queue_unlock_irqrestore(fqueue, flags);
>
> Is it possible to move this lock/list_add/unlock code chunk out of loop
> and before the folios_put(). I think it would be possible if you tag the
> corresponding index or have a separate bool array. It is also reasonable
> to claim that the contention of this lock is not a concern for now.

Considering the code complexity, perhaps we could wait until contention
on this lock becomes a problem?

Thanks,
Qi

>
>> }
>> + 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
>>