Re: [PATCH 2/2] mm/mglru: stop rotating offlined memcgs to the old generation

[Balbir Singh]

On Tue, Oct 27, 2026 at 06:06:06PM -0700, T.J. Mercier wrote:
> lru_gen_offline_memcg() moves an offlining memcg to the head of the old
> generation of the memcg LRU (MEMCG_LRU_OLD), so that global reclaim
> visits it first and evicts the folios it left behind.
> 
> Since commit f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of
> dying memory cgroup for LRU folios"), those folios are reparented to the
> parent before lru_gen_offline_memcg() runs, and nothing can be added
> back to the offlined memcg's lruvec afterwards. Putting an empty
> offlined memcg at the head of the old generation no longer serves any
> purpose and only delays reclaim from the memcgs that still have memory
> due to additional shrink_many() iteration.
> 
> Remove lru_gen_offline_memcg(), so that an offlined memcg is rotated
> like any other memcg below the reclaimable size threshold until
> lru_gen_release_memcg() takes it off the memcg LRU. MEMCG_LRU_OLD has no
> other user, so remove the op and update the memcg LRU comment as well.
> 
> Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
> ---
>  Documentation/mm/multigen_lru.rst |  4 +---
>  include/linux/mmzone.h            | 14 +++-----------
>  mm/memcontrol.c                   |  1 -
>  mm/vmscan.c                       | 20 +-------------------
>  4 files changed, 5 insertions(+), 34 deletions(-)
> 
> diff --git a/Documentation/mm/multigen_lru.rst b/Documentation/mm/multigen_lru.rst
> index 52ed5092022f..b532ce961371 100644
> --- a/Documentation/mm/multigen_lru.rst
> +++ b/Documentation/mm/multigen_lru.rst
> @@ -234,9 +234,7 @@ the active/inactive LRU (of folios):
>  1. It has the young and the old (generations), i.e., the counterparts
>     to the active and the inactive;
>  2. The increment of ``max_seq`` triggers promotion, i.e., the
> -   counterpart to activation;
> -3. Other events trigger similar operations, e.g., offlining an memcg
> -   triggers demotion, i.e., the counterpart to deactivation.
> +   counterpart to activation.
>  
>  In terms of global reclaim, it has two distinct features:
>  
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 7f078ff0dd76..542e465ee7a1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -626,12 +626,10 @@ struct lru_gen_mm_walk {
>   * per-node memcg generation counter, whose reminder (mod MEMCG_NR_GENS) indexes
>   * the old generation, is incremented when all its bins become empty.
>   *
> - * There are three operations:
> + * There are two operations:
>   * 1. MEMCG_LRU_TAIL, which moves a memcg to the tail of a random bin in its
>   *    current generation (old or young) and updates its "seg" to "tail";
> - * 2. MEMCG_LRU_OLD, which moves a memcg to the head of a random bin in the old
> - *    generation, updates its "gen" to "old" and resets its "seg" to "default";
> - * 3. MEMCG_LRU_YOUNG, which moves a memcg to the tail of a random bin in the
> + * 2. MEMCG_LRU_YOUNG, which moves a memcg to the tail of a random bin in the
>   *    young generation, updates its "gen" to "young" and resets its "seg" to
>   *    "default".
>   *
> @@ -643,8 +641,7 @@ struct lru_gen_mm_walk {
>   * 3. The second attempt to reclaim a memcg below reclaimable size threshold,
>   *    which triggers MEMCG_LRU_YOUNG;
>   * 4. Attempting to reclaim a memcg below min, which triggers MEMCG_LRU_YOUNG;
> - * 5. Finishing the aging on the eviction path, which triggers MEMCG_LRU_YOUNG;
> - * 6. Offlining a memcg, which triggers MEMCG_LRU_OLD.
> + * 5. Finishing the aging on the eviction path, which triggers MEMCG_LRU_YOUNG.
>   *
>   * Notes:
>   * 1. Memcg LRU only applies to global reclaim, and the round-robin incrementing
> @@ -675,7 +672,6 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr);
>  void lru_gen_init_memcg(struct mem_cgroup *memcg);
>  void lru_gen_exit_memcg(struct mem_cgroup *memcg);
>  void lru_gen_online_memcg(struct mem_cgroup *memcg);
> -void lru_gen_offline_memcg(struct mem_cgroup *memcg);
>  void lru_gen_release_memcg(struct mem_cgroup *memcg);
>  void max_lru_gen_memcg(struct mem_cgroup *memcg, int nid);
>  bool recheck_lru_gen_max_memcg(struct mem_cgroup *memcg, int nid);
> @@ -709,10 +705,6 @@ static inline void lru_gen_online_memcg(struct mem_cgroup *memcg)
>  {
>  }
>  
> -static inline void lru_gen_offline_memcg(struct mem_cgroup *memcg)
> -{
> -}
> -
>  static inline void lru_gen_release_memcg(struct mem_cgroup *memcg)
>  {
>  }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a05d9a8baa1b..e2ddf0680e62 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4475,7 +4475,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
>  	memcg_reparent_objcgs(memcg);
>  	reparent_shrinker_deferred(memcg);
>  	wb_memcg_offline(memcg);
> -	lru_gen_offline_memcg(memcg);
>  
>  	drain_all_stock(memcg);
>  
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 636152d8c94d..45a2c1021f4f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4538,7 +4538,6 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr)
>  enum {
>  	MEMCG_LRU_NOP,
>  	MEMCG_LRU_TAIL,
> -	MEMCG_LRU_OLD,
>  	MEMCG_LRU_YOUNG,
>  };
>  
> @@ -4560,8 +4559,6 @@ static void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
>  	/* see the comment on MEMCG_NR_GENS */
>  	if (op == MEMCG_LRU_TAIL)
>  		seg = MEMCG_LRU_TAIL;
> -	else if (op == MEMCG_LRU_OLD)
> -		new = get_memcg_gen(pgdat->memcg_lru.seq);
>  	else if (op == MEMCG_LRU_YOUNG)
>  		new = get_memcg_gen(pgdat->memcg_lru.seq + 1);
>  	else
> @@ -4571,11 +4568,7 @@ static void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
>  	WRITE_ONCE(lruvec->lrugen.gen, new);
>  
>  	hlist_nulls_del_rcu(&lruvec->lrugen.list);
> -
> -	if (op == MEMCG_LRU_OLD)
> -		hlist_nulls_add_head_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
> -	else
> -		hlist_nulls_add_tail_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
> +	hlist_nulls_add_tail_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
>  
>  	pgdat->memcg_lru.nr_memcgs[old]--;
>  	pgdat->memcg_lru.nr_memcgs[new]++;
> @@ -4613,17 +4606,6 @@ void lru_gen_online_memcg(struct mem_cgroup *memcg)
>  	}
>  }
>  
> -void lru_gen_offline_memcg(struct mem_cgroup *memcg)
> -{
> -	int nid;
> -
> -	for_each_node(nid) {
> -		struct lruvec *lruvec = get_lruvec(memcg, nid);
> -
> -		lru_gen_rotate_memcg(lruvec, MEMCG_LRU_OLD);
> -	}
> -}
> -
>  void lru_gen_release_memcg(struct mem_cgroup *memcg)
>  {
>  	int gen;

Acked-by: Balbir Singh <balbirs@xxxxxxxxxx>