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Message-ID: <9b9057f8-4c4c-4067-b6ba-0791888c25e8@linux.dev>
Date: Mon, 19 Jan 2026 11:34:53 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Shakeel Butt <shakeel.butt@...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 1/18/26 11:20 AM, Shakeel Butt wrote:
> 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/
OK, will do.
>
> 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
OK, will do.
>
>
>>
>> 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().
Since these two functions require memcg or lruvec, they are already
within the critical section of the RCU lock.
>
> 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:
It does look much simpler, will do.
Thanks,
Qi
>
> 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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