Re: [PATCH v1 07/26] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio()

Harry Yoo
2025-11-19T08:06:59Z

On Tue, Oct 28, 2025 at 09:58:20PM +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>
> ---

This patch looks correct to me, so:
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

with a few side question on current implementation rather than
this change...

> mm/memcontrol.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d484b632c790f..1da3ad77054d3 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)))

I have no idea why we're already calling rcu_read_lock() when a folio pins
the memcg :)

> - memcg = root_mem_cgroup;
> +retry:
> + memcg = folio_memcg(folio);
> + if (unlikely(!css_tryget(&memcg->css)))
> + goto retry;
> rcu_read_unlock();
> return memcg;
> }
> --
> 2.20.1

--
Cheers,
Harry / Hyeonggon