Re: [PATCH v5 update 29/32] mm: memcontrol: prepare for reparenting non-hierarchical stats

[Qi Zheng <qi.zheng@linux.dev>]

Hi Andrew,

On 3/1/26 3:08 AM, Andrew Morton wrote:
> On Sat, 28 Feb 2026 15:25:56 +0800 Qi Zheng <qi.zheng@linux.dev> wrote:
> 
>> To resolve the dying memcg issue, we need to reparent LRU folios of child
>> memcg to its parent memcg. This could cause problems for non-hierarchical
>> stats.
>>
>> As Yosry Ahmed pointed out:
>>
>> ```
>> In short, if memory is charged to a dying cgroup at the time of
>> reparenting, when the memory gets uncharged the stats updates will occur
>> at the parent. This will update both hierarchical and non-hierarchical
>> stats of the parent, which would corrupt the parent's non-hierarchical
>> stats (because those counters were never incremented when the memory was
>> charged).
>> ```
>>
>> Now we have the following two types of non-hierarchical stats, and they
>> are only used in CONFIG_MEMCG_V1:
>>
>> a. memcg->vmstats->state_local[i]
>> b. pn->lruvec_stats->state_local[i]
>>
>> To ensure that these non-hierarchical stats work properly, we need to
>> reparent these non-hierarchical stats after reparenting LRU folios. To
>> this end, this commit makes the following preparations:
>>
>> 1. implement reparent_state_local() to reparent non-hierarchical stats
>> 2. make css_killed_work_fn() to be called in rcu work, and implement
>>     get_non_dying_memcg_start() and get_non_dying_memcg_end() to avoid race
>>     between mod_memcg_state()/mod_memcg_lruvec_state()
>>     and reparent_state_local()
> 
> That's a big update (delta patch is below).
> 
> What did it all do?

Oh sorry, I forgot the changlog.

> 
> I've bookmarked these reviewer comments:
> 
> https://lkml.kernel.org/r/CAO9r8zOhZgrym6oSrtg7b+HmNHEfWuAzZ0i8eYhm5-OEnfFLdw@mail.gmail.com
> https://lkml.kernel.org/r/aZ-R87JfacQ2gGq1@linux.dev

Based on Yosry Ahmed's suggestion and diff, the method has been changed
to non-atomic mode, so there is no need to test the perfomance impact of
atomic mode.

> https://lkml.kernel.org/r/aZ-kefGBeT-RzGcG@linux.dev

This redundant declaration has been removed.

Thanks,
Qi

> 
> Were they all addressed in some fashion?
> 
> 
>   include/linux/memcontrol.h |    1
>   mm/memcontrol-v1.h         |    1
>   mm/memcontrol.c            |   89 ++++++++++++++++++++++++++---------
>   3 files changed, 68 insertions(+), 23 deletions(-)
> 
> --- a/include/linux/memcontrol.h~mm-memcontrol-prepare-for-reparenting-non-hierarchical-stats-update
> +++ a/include/linux/memcontrol.h
> @@ -956,7 +956,6 @@ static inline void mod_memcg_page_state(
>   
>   unsigned long memcg_events(struct mem_cgroup *memcg, int event);
>   unsigned long memcg_page_state(struct mem_cgroup *memcg, 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);
> --- a/mm/memcontrol.c~mm-memcontrol-prepare-for-reparenting-non-hierarchical-stats-update
> +++ a/mm/memcontrol.c
> @@ -234,11 +234,19 @@ static inline void reparent_state_local(
>   	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
>   		return;
>   
> +	/*
> +	 * Reparent stats exposed non-hierarchically. Flush @memcg's stats first
> +	 * to read its stats accurately , and conservatively flush @parent's
> +	 * stats after reparenting to avoid hiding a potentially large stat
> +	 * update (e.g. from callers of mem_cgroup_flush_stats_ratelimited()).
> +	 */
>   	__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);
> +
> +	__mem_cgroup_flush_stats(parent, true);
>   }
>   #else
>   static inline void reparent_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent)
> @@ -442,7 +450,7 @@ struct lruvec_stats {
>   	long state[NR_MEMCG_NODE_STAT_ITEMS];
>   
>   	/* Non-hierarchical (CPU aggregated) state */
> -	atomic_long_t state_local[NR_MEMCG_NODE_STAT_ITEMS];
> +	long state_local[NR_MEMCG_NODE_STAT_ITEMS];
>   
>   	/* Pending child counts during tree propagation */
>   	long state_pending[NR_MEMCG_NODE_STAT_ITEMS];
> @@ -485,7 +493,7 @@ unsigned long lruvec_page_state_local(st
>   		return 0;
>   
>   	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> -	x = atomic_long_read(&(pn->lruvec_stats->state_local[i]));
> +	x = READ_ONCE(pn->lruvec_stats->state_local[i]);
>   #ifdef CONFIG_SMP
>   	if (x < 0)
>   		x = 0;
> @@ -494,6 +502,9 @@ unsigned long lruvec_page_state_local(st
>   }
>   
>   #ifdef CONFIG_MEMCG_V1
> +static void __mod_memcg_lruvec_state(struct lruvec *lruvec,
> +				     enum node_stat_item idx, int val);
> +
>   void reparent_memcg_lruvec_state_local(struct mem_cgroup *memcg,
>   				       struct mem_cgroup *parent, int idx)
>   {
> @@ -506,12 +517,10 @@ void reparent_memcg_lruvec_state_local(s
>   	for_each_node(nid) {
>   		struct lruvec *child_lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(nid));
>   		struct lruvec *parent_lruvec = mem_cgroup_lruvec(parent, NODE_DATA(nid));
> -		struct mem_cgroup_per_node *parent_pn;
>   		unsigned long value = lruvec_page_state_local(child_lruvec, idx);
>   
> -		parent_pn = container_of(parent_lruvec, struct mem_cgroup_per_node, lruvec);
> -
> -		atomic_long_add(value, &(parent_pn->lruvec_stats->state_local[i]));
> +		__mod_memcg_lruvec_state(child_lruvec, idx, -value);
> +		__mod_memcg_lruvec_state(parent_lruvec, idx, value);
>   	}
>   }
>   #endif
> @@ -598,7 +607,7 @@ struct memcg_vmstats {
>   	unsigned long		events[NR_MEMCG_EVENTS];
>   
>   	/* Non-hierarchical (CPU aggregated) page state & events */
> -	atomic_long_t		state_local[MEMCG_VMSTAT_SIZE];
> +	long			state_local[MEMCG_VMSTAT_SIZE];
>   	unsigned long		events_local[NR_MEMCG_EVENTS];
>   
>   	/* Pending child counts during tree propagation */
> @@ -835,7 +844,7 @@ unsigned long memcg_page_state_local(str
>   	if (WARN_ONCE(BAD_STAT_IDX(i), "%s: missing stat item %d\n", __func__, idx))
>   		return 0;
>   
> -	x = atomic_long_read(&(memcg->vmstats->state_local[i]));
> +	x = READ_ONCE(memcg->vmstats->state_local[i]);
>   #ifdef CONFIG_SMP
>   	if (x < 0)
>   		x = 0;
> @@ -843,6 +852,51 @@ unsigned long memcg_page_state_local(str
>   	return x;
>   }
>   
> +static void __mod_memcg_state(struct mem_cgroup *memcg,
> +			      enum memcg_stat_item idx, int val)
> +{
> +	int i = memcg_stats_index(idx);
> +	int cpu;
> +
> +	if (mem_cgroup_disabled())
> +		return;
> +
> +	cpu = get_cpu();
> +
> +	this_cpu_add(memcg->vmstats_percpu->state[i], val);
> +	val = memcg_state_val_in_pages(idx, val);
> +	memcg_rstat_updated(memcg, val, cpu);
> +	trace_mod_memcg_state(memcg, idx, val);
> +
> +	put_cpu();
> +}
> +
> +static void __mod_memcg_lruvec_state(struct lruvec *lruvec,
> +				     enum node_stat_item idx, int val)
> +{
> +	struct mem_cgroup_per_node *pn;
> +	struct mem_cgroup *memcg;
> +	int i = memcg_stats_index(idx);
> +	int cpu;
> +
> +	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> +	memcg = pn->memcg;
> +
> +	cpu = get_cpu();
> +
> +	/* Update memcg */
> +	this_cpu_add(memcg->vmstats_percpu->state[i], val);
> +
> +	/* Update lruvec */
> +	this_cpu_add(pn->lruvec_stats_percpu->state[i], val);
> +
> +	val = memcg_state_val_in_pages(idx, val);
> +	memcg_rstat_updated(memcg, val, cpu);
> +	trace_mod_memcg_lruvec_state(memcg, idx, val);
> +
> +	put_cpu();
> +}
> +
>   void reparent_memcg_state_local(struct mem_cgroup *memcg,
>   				struct mem_cgroup *parent, int idx)
>   {
> @@ -852,7 +906,8 @@ void reparent_memcg_state_local(struct m
>   	if (WARN_ONCE(BAD_STAT_IDX(i), "%s: missing stat item %d\n", __func__, idx))
>   		return;
>   
> -	atomic_long_add(value, &(parent->vmstats->state_local[i]));
> +	__mod_memcg_state(memcg, idx, -value);
> +	__mod_memcg_state(parent, idx, value);
>   }
>   #endif
>   
> @@ -4174,8 +4229,6 @@ struct aggregate_control {
>   	long *aggregate;
>   	/* pointer to the non-hierarchichal (CPU aggregated) counters */
>   	long *local;
> -	/* pointer to the atomic non-hierarchichal (CPU aggregated) counters */
> -	atomic_long_t *alocal;
>   	/* pointer to the pending child counters during tree propagation */
>   	long *pending;
>   	/* pointer to the parent's pending counters, could be NULL */
> @@ -4213,12 +4266,8 @@ static void mem_cgroup_stat_aggregate(st
>   		}
>   
>   		/* Aggregate counts on this level and propagate upwards */
> -		if (delta_cpu) {
> -			if (ac->local)
> -				ac->local[i] += delta_cpu;
> -			else if (ac->alocal)
> -				atomic_long_add(delta_cpu, &(ac->alocal[i]));
> -		}
> +		if (delta_cpu)
> +			ac->local[i] += delta_cpu;
>   
>   		if (delta) {
>   			ac->aggregate[i] += delta;
> @@ -4289,8 +4338,7 @@ static void mem_cgroup_css_rstat_flush(s
>   
>   	ac = (struct aggregate_control) {
>   		.aggregate = memcg->vmstats->state,
> -		.local = NULL,
> -		.alocal = memcg->vmstats->state_local,
> +		.local = memcg->vmstats->state_local,
>   		.pending = memcg->vmstats->state_pending,
>   		.ppending = parent ? parent->vmstats->state_pending : NULL,
>   		.cstat = statc->state,
> @@ -4323,8 +4371,7 @@ static void mem_cgroup_css_rstat_flush(s
>   
>   		ac = (struct aggregate_control) {
>   			.aggregate = lstats->state,
> -			.local = NULL,
> -			.alocal = lstats->state_local,
> +			.local = lstats->state_local,
>   			.pending = lstats->state_pending,
>   			.ppending = plstats ? plstats->state_pending : NULL,
>   			.cstat = lstatc->state,
> --- a/mm/memcontrol-v1.h~mm-memcontrol-prepare-for-reparenting-non-hierarchical-stats-update
> +++ a/mm/memcontrol-v1.h
> @@ -45,7 +45,6 @@ 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);
> _
>