Re: [PATCH v2 27/28] mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Dec 17, 2025 at 03:27:51PM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@bytedance.com>
> 
> Pagecache pages are charged at allocation time and hold a reference
> to the original memory cgroup until reclaimed. Depending on memory
> pressure, page sharing patterns between different cgroups and cgroup
> creation/destruction rates, many dying memory cgroups can be pinned
> by pagecache pages, reducing page reclaim efficiency and wasting
> memory. Converting LRU folios and most other raw memory cgroup pins
> to the object cgroup direction can fix this long-living problem.

This is already in the coverletter. Please describe here what the
patch itself does. IOW, now that everything is set up, switch
folio->memcg_data pointers to objcgs, update the accessors, and
execute reparenting on cgroup death.

> Finally, folio->memcg_data of LRU folios and kmem folios will always
> point to an object cgroup pointer. The folio->memcg_data of slab
> folios will point to an vector of object cgroups.

> @@ -223,22 +223,55 @@ static inline void __memcg_reparent_objcgs(struct mem_cgroup *src,
>  
>  static inline void reparent_locks(struct mem_cgroup *src, struct mem_cgroup *dst)
>  {
> +	int nid, nest = 0;
> +
>  	spin_lock_irq(&objcg_lock);
> +	for_each_node(nid) {
> +		spin_lock_nested(&mem_cgroup_lruvec(src,
> +				 NODE_DATA(nid))->lru_lock, nest++);
> +		spin_lock_nested(&mem_cgroup_lruvec(dst,
> +				 NODE_DATA(nid))->lru_lock, nest++);
> +	}
>  }

Looks okay to me. If this should turn out to be a scalability problem
in practice, we can make objcgs per-node, and then reparent lru/objcg
pairs on a per-node basis without nesting locks.

>  static inline void reparent_unlocks(struct mem_cgroup *src, struct mem_cgroup *dst)
>  {
> +	int nid;
> +
> +	for_each_node(nid) {
> +		spin_unlock(&mem_cgroup_lruvec(dst, NODE_DATA(nid))->lru_lock);
> +		spin_unlock(&mem_cgroup_lruvec(src, NODE_DATA(nid))->lru_lock);
> +	}
>  	spin_unlock_irq(&objcg_lock);
>  }
>  
> +static void memcg_reparent_lru_folios(struct mem_cgroup *src,
> +				      struct mem_cgroup *dst)
> +{
> +	if (lru_gen_enabled())
> +		lru_gen_reparent_memcg(src, dst);
> +	else
> +		lru_reparent_memcg(src, dst);
> +}
> +
>  static void memcg_reparent_objcgs(struct mem_cgroup *src)
>  {
>  	struct obj_cgroup *objcg = rcu_dereference_protected(src->objcg, true);
>  	struct mem_cgroup *dst = parent_mem_cgroup(src);
>  
> +retry:
> +	if (lru_gen_enabled())
> +		max_lru_gen_memcg(dst);
> +
>  	reparent_locks(src, dst);
> +	if (lru_gen_enabled() && !recheck_lru_gen_max_memcg(dst)) {
> +		reparent_unlocks(src, dst);
> +		cond_resched();
> +		goto retry;
> +	}
>  
>  	__memcg_reparent_objcgs(src, dst);
> +	memcg_reparent_lru_folios(src, dst);

Please inline memcg_reparent_lru_folios() here, to keep the lru vs
lrugen switching as "flat" as possible:

	if (lru_gen_enabled()) {
		if (!recheck_lru_gen_max_memcgs(parent)) {
			reparent_unlocks(memcg, parent);
			cond_resched();
			goto retry;
		}
		lru_gen_reparent_memcg(memcg, parent);
	} else {
		lru_reparent_memcg(memcg, parent);
	}

> @@ -989,6 +1022,8 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
>  /**
>   * get_mem_cgroup_from_folio - Obtain a reference on a given folio's memcg.
>   * @folio: folio from which memcg should be extracted.
> + *
> + * The folio and objcg or memcg binding rules can refer to folio_memcg().

      See folio_memcg() for folio->objcg/memcg binding rules.