memcg: drain obj stock on cpu hotplug teardown

commit 9f01b49
Author: Shakeel Butt <[email protected]>
Date:   Mon Mar 10 16:09:34 2025 -0700

    memcg: drain obj stock on cpu hotplug teardown

    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/[email protected]
    Fixes: bf4f059 ("mm: memcg/slab: obj_cgroup API")
    Signed-off-by: Shakeel Butt <[email protected]>
    Reviewed-by: Roman Gushchin <[email protected]>
    Acked-by: Johannes Weiner <[email protected]>
    Cc: Michal Hocko <[email protected]>
    Cc: Muchun Song <[email protected]>
    Cc: <[email protected]>
    Signed-off-by: Andrew Morton <[email protected]>

JIRA: https://issues.redhat.com/browse/RHEL-77742
Signed-off-by: Nico Pache <[email protected]>

Author: PacheNico

mm/memcontrol.c

@@ -1938,9 +1938,18 @@ 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;
 }