[SRU][N][PATCH 2/2] memcg: drain obj stock on cpu hotplug teardown

[Juerg Haefliger]

From: Shakeel Butt <shakeel.butt at linux.dev>

BugLink: https://bugs.launchpad.net/bugs/2121673

commit 9f01b4954490d4ccdbcc2b9be34a9921ceee9cbb upstream.

Currently on cpu hotplug teardown, only memcg stock is drained but we
need to drain the obj stock as well otherwise we will miss the stats
accumulated on the target cpu as well as the nr_bytes cached. The stats
include MEMCG_KMEM, NR_SLAB_RECLAIMABLE_B & NR_SLAB_UNRECLAIMABLE_B. In
addition we are leaking reference to struct obj_cgroup object.

Link: https://lkml.kernel.org/r/20250310230934.2913113-1-shakeel.butt@linux.dev
Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Shakeel Butt <shakeel.butt at linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin at linux.dev>
Acked-by: Johannes Weiner <hannes at cmpxchg.org>
Cc: Michal Hocko <mhocko at kernel.org>
Cc: Muchun Song <muchun.song at linux.dev>
Cc: <stable at vger.kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit 9becd7c25c61ae7e5b6fbfc3c226b1f23af7638c linux-6.12.y)
Signed-off-by: Juerg Haefliger <juerg.haefliger at canonical.com>
---
 mm/memcontrol.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 33dc2bad0619..8a1aecb0d653 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2453,9 +2453,18 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 static int memcg_hotplug_cpu_dead(unsigned int cpu)
 {
 	struct memcg_stock_pcp *stock;
+	struct obj_cgroup *old;
+	unsigned long flags;
 
 	stock = &per_cpu(memcg_stock, cpu);
+
+	/* drain_obj_stock requires stock_lock */
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
+	old = drain_obj_stock(stock);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
+
 	drain_stock(stock);
+	obj_cgroup_put(old);
 
 	return 0;
 }
-- 
2.48.1