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Message-ID: <iu27pt5nqs6myshw57e7dotld33v6lwuyouvquoqc2zmc5loi6@f23auf7hqbdp>
Date: Sat, 17 Jan 2026 19:20:42 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...nel.org,
lorenzo.stoakes@...cle.com, ziy@...dia.com, harry.yoo@...cle.com, yosry.ahmed@...ux.dev,
imran.f.khan@...cle.com, kamalesh.babulal@...cle.com, axelrasmussen@...gle.com,
yuanchu@...gle.com, weixugc@...gle.com, chenridong@...weicloud.com, mkoutny@...e.com,
akpm@...ux-foundation.org, hamzamahfooz@...ux.microsoft.com, apais@...ux.microsoft.com,
lance.yang@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
cgroups@...r.kernel.org, Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v3 28/30] mm: memcontrol: prepare for reparenting
state_local
On Wed, Jan 14, 2026 at 07:32:55PM +0800, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@...edance.com>
>
> To resolve the dying memcg issue, we need to reparent LRU folios of child
> memcg to its parent memcg. The following counts are all non-hierarchical
> and need to be reparented to prevent the counts of parent memcg overflow.
>
> 1. memcg->vmstats->state_local[i]
> 2. pn->lruvec_stats->state_local[i]
>
> This commit implements the specific function, which will be used during
> the reparenting process.
Please add more explanation which was discussed in the email chain at
https://lore.kernel.org/all/5dsb6q2r4xsi24kk5gcnckljuvgvvp6nwifwvc4wuho5hsifeg@5ukg2dq6ini5/
Also move the upward traversal code in mod_memcg_state() and
mod_memcg_lruvec_state() you added in later patch to this patch and make
it under CONFIG_MEMCG_V1.
Something like:
#ifdef CONFIG_MEMCG_V1
while (memcg_is_dying(memcg))
memcg = parent_mem_cgroup(memcg);
#endif
>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
> include/linux/memcontrol.h | 4 +++
> mm/memcontrol-v1.c | 16 +++++++++++
> mm/memcontrol-v1.h | 3 ++
> mm/memcontrol.c | 56 ++++++++++++++++++++++++++++++++++++++
> 4 files changed, 79 insertions(+)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 26c3c0e375f58..f84a23f13ffb4 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -963,12 +963,16 @@ static inline void mod_memcg_page_state(struct page *page,
>
> unsigned long memcg_events(struct mem_cgroup *memcg, int event);
> unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx);
> +void reparent_memcg_state_local(struct mem_cgroup *memcg,
> + struct mem_cgroup *parent, int idx);
> unsigned long memcg_page_state_output(struct mem_cgroup *memcg, int item);
> bool memcg_stat_item_valid(int idx);
> bool memcg_vm_event_item_valid(enum vm_event_item idx);
> unsigned long lruvec_page_state(struct lruvec *lruvec, enum node_stat_item idx);
> unsigned long lruvec_page_state_local(struct lruvec *lruvec,
> enum node_stat_item idx);
> +void reparent_memcg_lruvec_state_local(struct mem_cgroup *memcg,
> + struct mem_cgroup *parent, int idx);
>
> void mem_cgroup_flush_stats(struct mem_cgroup *memcg);
> void mem_cgroup_flush_stats_ratelimited(struct mem_cgroup *memcg);
> diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
> index f0ef650d2317b..800606135e7ba 100644
> --- a/mm/memcontrol-v1.c
> +++ b/mm/memcontrol-v1.c
> @@ -1898,6 +1898,22 @@ static const unsigned int memcg1_events[] = {
> PGMAJFAULT,
> };
>
> +void reparent_memcg1_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent)
> +{
> + int i;
> +
> + for (i = 0; i < ARRAY_SIZE(memcg1_stats); i++)
> + reparent_memcg_state_local(memcg, parent, memcg1_stats[i]);
> +}
> +
> +void reparent_memcg1_lruvec_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent)
> +{
> + int i;
> +
> + for (i = 0; i < ARRAY_SIZE(memcg1_stats); i++)
> + reparent_memcg_lruvec_state_local(memcg, parent, memcg1_stats[i]);
> +}
> +
> void memcg1_stat_format(struct mem_cgroup *memcg, struct seq_buf *s)
> {
> unsigned long memory, memsw;
> diff --git a/mm/memcontrol-v1.h b/mm/memcontrol-v1.h
> index eb3c3c1056574..45528195d3578 100644
> --- a/mm/memcontrol-v1.h
> +++ b/mm/memcontrol-v1.h
> @@ -41,6 +41,7 @@ static inline bool do_memsw_account(void)
>
> unsigned long memcg_events_local(struct mem_cgroup *memcg, int event);
> unsigned long memcg_page_state_local(struct mem_cgroup *memcg, int idx);
> +void mod_memcg_page_state_local(struct mem_cgroup *memcg, int idx, unsigned long val);
> unsigned long memcg_page_state_local_output(struct mem_cgroup *memcg, int item);
> bool memcg1_alloc_events(struct mem_cgroup *memcg);
> void memcg1_free_events(struct mem_cgroup *memcg);
> @@ -73,6 +74,8 @@ void memcg1_uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
> unsigned long nr_memory, int nid);
>
> void memcg1_stat_format(struct mem_cgroup *memcg, struct seq_buf *s);
> +void reparent_memcg1_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent);
> +void reparent_memcg1_lruvec_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent);
>
> void memcg1_account_kmem(struct mem_cgroup *memcg, int nr_pages);
> static inline bool memcg1_tcpmem_active(struct mem_cgroup *memcg)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 70583394f421f..7aa32b97c9f17 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -225,6 +225,28 @@ static inline struct obj_cgroup *__memcg_reparent_objcgs(struct mem_cgroup *memc
> return objcg;
> }
>
> +#ifdef CONFIG_MEMCG_V1
> +static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force);
> +
> +static inline void reparent_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent)
> +{
> + if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> + return;
> +
> + synchronize_rcu();
Hmm synchrinuze_rcu() is a heavy hammer here. Also you would need rcu
read lock in mod_memcg_state() & mod_memcg_lruvec_state() for this
synchronize_rcu().
Hmm instead of synchronize_rcu() here, we can use queue_rcu_work() in
css_killed_ref_fn(). It would be as simple as the following:
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index e717208cfb18..549a8e026194 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6046,8 +6046,8 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
*/
static void css_killed_work_fn(struct work_struct *work)
{
- struct cgroup_subsys_state *css =
- container_of(work, struct cgroup_subsys_state, destroy_work);
+ struct cgroup_subsys_state *css = container_of(to_rcu_work(work),
+ struct cgroup_subsys_state, destroy_rwork);
cgroup_lock();
@@ -6068,8 +6068,8 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
container_of(ref, struct cgroup_subsys_state, refcnt);
if (atomic_dec_and_test(&css->online_cnt)) {
- INIT_WORK(&css->destroy_work, css_killed_work_fn);
- queue_work(cgroup_offline_wq, &css->destroy_work);
+ INIT_RCU_WORK(&css->destroy_rwork, css_killed_work_fn);
+ queue_rcu_work(cgroup_offline_wq, &css->destroy_rwork);
}
}
> +
> + __mem_cgroup_flush_stats(memcg, true);
> +
> + /* The following counts are all non-hierarchical and need to be reparented. */
> + reparent_memcg1_state_local(memcg, parent);
> + reparent_memcg1_lruvec_state_local(memcg, parent);
> +}
> +#else
> +static inline void reparent_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent)
> +{
> +}
> +#endif
> +
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