Re: [PATCH v2 08/28] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio()

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Dec 17, 2025 at 03:27:32PM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@bytedance.com>
> 
> 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 get_mem_cgroup_from_folio().
> 
> This serves as a preparatory measure for the reparenting of the
> LRU pages.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  mm/memcontrol.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 21b5aad34cae7..431b3154c70c5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -973,14 +973,19 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
>   */
>  struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
>  {
> -	struct mem_cgroup *memcg = folio_memcg(folio);
> +	struct mem_cgroup *memcg;
>  
>  	if (mem_cgroup_disabled())
>  		return NULL;
>  
> +	if (!folio_memcg_charged(folio))
> +		return root_mem_cgroup;
> +
>  	rcu_read_lock();
> -	if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
> -		memcg = root_mem_cgroup;
> +retry:
> +	memcg = folio_memcg(folio);
> +	if (unlikely(!css_tryget(&memcg->css)))
> +		goto retry;

So starting in patch 27, the tryget can fail if the memcg is offlined,
and the folio's objcg is reparented concurrently. We'll retry until we
find a memcg that isn't dead yet. There's always root_mem_cgroup.

It makes sense, but a loop like this begs the question of how it is
bounded. I pieced it together looking ahead. Since this is a small
diff, it would be nicer to fold it into 27. I didn't see anything in
between depending on it, but correct me if I'm wrong.

Minor style preference:

	/* Comment explaining the above */
	do {
		memcg = folio_memcg(folio);
	} while (!css_tryget(&memcg->css));