linux-kernel - Re: [PATCH v5 3/4] mm: thp: use folio_batch to handle THP splitting in deferred_split

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Message-ID: <20251017004611.ccjq2343v43mimqq@master>
Date: Fri, 17 Oct 2025 00:46:11 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
	roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
	muchun.song@...ux.dev, david@...hat.com, lorenzo.stoakes@...cle.com,
	ziy@...dia.com, harry.yoo@...cle.com, baolin.wang@...ux.alibaba.com,
	Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
	dev.jain@....com, baohua@...nel.org, lance.yang@...ux.dev,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	Muchun Song <songmuchun@...edance.com>,
	Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v5 3/4] mm: thp: use folio_batch to handle THP splitting
 in deferred_split_scan()

On Wed, Oct 15, 2025 at 02:35:32PM +0800, Qi Zheng wrote:
>From: Muchun Song <songmuchun@...edance.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,
>      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@...edance.com>
>Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>Reviewed-by: Zi Yan <ziy@...dia.com>
>Acked-by: David Hildenbrand <david@...hat.com>
>Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>

Reviewed-by: Wei Yang <richard.weiyang@...il.com>

One nit below

>---
[...]
>@@ -4239,38 +4245,27 @@ 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);
> 	}
>+	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 && !list_empty(&ds_queue->split_queue)) {

Maybe we can use ds_queue->split_queue_len instead?

>+		cond_resched();
>+		goto retry;
>+	}
> 
> 	/*
> 	 * Stop shrinker if we didn't split any page, but the queue is empty.
>-- 
>2.20.1
>

-- 
Wei Yang
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