Re: [PATCH v2 15/28] mm: memcontrol: prevent memory cgroup release in mem_cgroup_swap_full()

[Shakeel Butt]

On Wed, Dec 17, 2025 at 03:27:39PM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> 
> In the near future, a folio will no longer pin its corresponding
> memory cgroup. To ensure safety, it will only be appropriate to
> hold the rcu read lock or acquire a reference to the memory cgroup
> returned by folio_memcg(), thereby preventing it from being released.
> 
> In the current patch, the rcu read lock is employed to safeguard
> against the release of the memory cgroup in mem_cgroup_swap_full().
> 
> This serves as a preparatory measure for the reparenting of the
> LRU pages.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> ---
>  mm/memcontrol.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 131f940c03fa0..f2c891c1f49d5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5267,17 +5267,21 @@ bool mem_cgroup_swap_full(struct folio *folio)
>  	if (do_memsw_account())
>  		return false;
>  
> -	memcg = folio_memcg(folio);
> -	if (!memcg)
> +	if (!folio_memcg_charged(folio))
>  		return false;
>  
> +	rcu_read_lock();
> +	memcg = folio_memcg(folio);
>  	for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
>  		unsigned long usage = page_counter_read(&memcg->swap);
>  
>  		if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
> -		    usage * 2 >= READ_ONCE(memcg->swap.max))
> +		    usage * 2 >= READ_ONCE(memcg->swap.max)) {
> +			rcu_read_unlock();
>  			return true;
> +		}
>  	}
> +	rcu_read_unlock();
>  
>  	return false;
>  }

How about the following?


 bool mem_cgroup_swap_full(struct folio *folio)
 {
 	struct mem_cgroup *memcg;
+	bool ret = false;
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 
 	if (vm_swap_full())
 		return true;
-	if (do_memsw_account())
-		return false;
 
-	if (!folio_memcg_charged(folio))
-		return false;
+	if (do_memsw_account() || !folio_memcg_charged(folio))
+		return ret;
 
 	rcu_read_lock();
 	memcg = folio_memcg(folio);
@@ -5277,13 +5276,13 @@ bool mem_cgroup_swap_full(struct folio *folio)
 
 		if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
 		    usage * 2 >= READ_ONCE(memcg->swap.max)) {
-			rcu_read_unlock();
-			return true;
+			ret = true;
+			break;
 		}
 	}
 	rcu_read_unlock();
 
-	return false;
+	return ret;
 }
 

Anyways LGTM.

Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>