Re: [PATCH] memcg: drain obj stock on cpu hotplug teardown

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Mar 10, 2025 at 04:09:34PM -0700, Shakeel Butt wrote:
> 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.
> 
> Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> Cc: <stable@vger.kernel.org>

Wow, that's old. Good catch.

> ---
>  mm/memcontrol.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4de6acb9b8ec..59dcaf6a3519 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1921,9 +1921,18 @@ 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);

It might be better to call drain_local_stock() directly instead. That
would prevent a bug of this type to reoccur in the future.