[PATCH RFC 07/28] mm: thp: use folio_batch to handle THP splitting in deferred_split_scan()

[Muchun Song]

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. 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 aim to reparent LRU folios during memcg offline to
eliminate dying memory cgroups. This patch prepares for using
folio_split_queue_lock_irqsave() as folio memcg may change then.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/huge_memory.c | 69 +++++++++++++++++++++---------------------------
 1 file changed, 30 insertions(+), 39 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 70820fa75c1f..d2bc943a40e8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4220,40 +4220,47 @@ 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;
+	bool done;
 
 #ifdef CONFIG_MEMCG
 	if (sc->memcg)
 		ds_queue = &sc->memcg->deferred_split_queue;
 #endif
-
+	folio_batch_init(&fbatch);
+retry:
+	done = true;
 	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--;
 		if (!--sc->nr_to_scan)
 			break;
+		if (folio_batch_space(&fbatch) == 0) {
+			done = false;
+			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)) {
 			underused = thp_underused(folio);
 			if (!underused)
@@ -4269,39 +4276,23 @@ 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);
-		}
-		if (folio)
-			folio_put(folio);
+		fqueue = folio_split_queue_lock_irqsave(folio, &flags);
+		list_add_tail(&folio->_deferred_list, &fqueue->split_queue);
+		fqueue->split_queue_len++;
+		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;
-	split_queue_unlock_irqrestore(ds_queue, flags);
-
-	if (prev)
-		folio_put(prev);
-
+	if (!done)
+		goto retry;
 	/*
 	 * Stop shrinker if we didn't split any page, but the queue is empty.
 	 * This can happen if pages were freed under us.
-- 
2.20.1