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

[Qi Zheng]

On 12/20/25 9:05 AM, Shakeel Butt wrote:
> 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;
>   }

LGTM, will do.

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

Thanks!