mm/thp: clear deferred split shrinker bits when queues drain

deferred_split_count() returns the raw list_lru count.  When the
per-memcg, per-node list is empty, that count is 0.

That skips scanning, but it does not tell memcg reclaim that the shrinker
is empty.  shrink_slab_memcg() only clears the memcg shrinker bit when the
count callback reports SHRINK_EMPTY.

Return SHRINK_EMPTY for an empty deferred split list, so the bit can be
cleared once the queue has drained.

Link: https://lore.kernel.org/20260602043453.67597-1-lance.yang@linux.dev
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Usama Arif <usama.arif@linux.dev>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nico Pache <npache@redhat.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -4410,7 +4410,10 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped)
 static unsigned long deferred_split_count(struct shrinker *shrink,
 		struct shrink_control *sc)
 {
-	return list_lru_shrink_count(&deferred_split_lru, sc);
+	unsigned long count;
+
+	count = list_lru_shrink_count(&deferred_split_lru, sc);
+	return count ?: SHRINK_EMPTY;
 }
 
 static bool thp_underused(struct folio *folio)