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

[Shakeel Butt <shakeel.butt@linux.dev>]

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.

> 
>    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>

> ---
>  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.

>  	}
> +	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
>