Re: [PATCH v3 26/30] mm: vmscan: prepare for reparenting MGLRU folios

[Shakeel Butt <shakeel.butt@linux.dev>]

Axel, Yuanchu & Wei, please help reviewing this patch.

On Wed, Jan 14, 2026 at 07:32:53PM +0800, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@bytedance.com>
> 
> Similar to traditional LRU folios, in order to solve the dying memcg
> problem, we also need to reparenting MGLRU folios to the parent memcg when
> memcg offline.
> 
> However, there are the following challenges:
> 
> 1. Each lruvec has between MIN_NR_GENS and MAX_NR_GENS generations, the
>    number of generations of the parent and child memcg may be different,
>    so we cannot simply transfer MGLRU folios in the child memcg to the
>    parent memcg as we did for traditional LRU folios.
> 2. The generation information is stored in folio->flags, but we cannot
>    traverse these folios while holding the lru lock, otherwise it may
>    cause softlockup.
> 3. In walk_update_folio(), the gen of folio and corresponding lru size
>    may be updated, but the folio is not immediately moved to the
>    corresponding lru list. Therefore, there may be folios of different
>    generations on an LRU list.
> 4. In lru_gen_del_folio(), the generation to which the folio belongs is
>    found based on the generation information in folio->flags, and the
>    corresponding LRU size will be updated. Therefore, we need to update
>    the lru size correctly during reparenting, otherwise the lru size may
>    be updated incorrectly in lru_gen_del_folio().
> 
> Finally, this patch chose a compromise method, which is to splice the lru
> list in the child memcg to the lru list of the same generation in the
> parent memcg during reparenting. And in order to ensure that the parent
> memcg has the same generation, we need to increase the generations in the
> parent memcg to the MAX_NR_GENS before reparenting.
> 
> Of course, the same generation has different meanings in the parent and
> child memcg, this will cause confusion in the hot and cold information of
> folios. But other than that, this method is simple enough, the lru size
> is correct, and there is no need to consider some concurrency issues (such
> as lru_gen_del_folio()).
> 
> To prepare for the above work, this commit implements the specific
> functions, which will be used during reparenting.
> 
> Suggested-by: Harry Yoo <harry.yoo@oracle.com>
> Suggested-by: Imran Khan <imran.f.khan@oracle.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>  include/linux/mmzone.h |  16 +++++
>  mm/vmscan.c            | 144 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 160 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 1014b5a93c09c..a41f4f0ae5eb7 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -628,6 +628,9 @@ 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 lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid);
> +void max_lru_gen_memcg(struct mem_cgroup *memcg);
> +bool recheck_lru_gen_max_memcg(struct mem_cgroup *memcg);
> +void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent);
>  
>  #else /* !CONFIG_LRU_GEN */
>  
> @@ -668,6 +671,19 @@ static inline void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid)
>  {
>  }
>  
> +static inline void max_lru_gen_memcg(struct mem_cgroup *memcg)
> +{
> +}
> +
> +static inline bool recheck_lru_gen_max_memcg(struct mem_cgroup *memcg)
> +{
> +	return true;
> +}
> +
> +static inline void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent)
> +{
> +}
> +
>  #endif /* CONFIG_LRU_GEN */
>  
>  struct lruvec {
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e738082874878..6bc8047b7aec5 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4445,6 +4445,150 @@ void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid)
>  		lru_gen_rotate_memcg(lruvec, MEMCG_LRU_HEAD);
>  }
>  
> +bool recheck_lru_gen_max_memcg(struct mem_cgroup *memcg)
> +{
> +	int nid;
> +
> +	for_each_node(nid) {
> +		struct lruvec *lruvec = get_lruvec(memcg, nid);
> +		int type;
> +
> +		for (type = 0; type < ANON_AND_FILE; type++) {
> +			if (get_nr_gens(lruvec, type) != MAX_NR_GENS)
> +				return false;
> +		}
> +	}
> +
> +	return true;
> +}
> +
> +static void try_to_inc_max_seq_nowalk(struct mem_cgroup *memcg,
> +				      struct lruvec *lruvec)
> +{
> +	struct lru_gen_mm_list *mm_list = get_mm_list(memcg);
> +	struct lru_gen_mm_state *mm_state = get_mm_state(lruvec);
> +	int swappiness = mem_cgroup_swappiness(memcg);
> +	DEFINE_MAX_SEQ(lruvec);
> +	bool success = false;
> +
> +	/*
> +	 * We are not iterating the mm_list here, updating mm_state->seq is just
> +	 * to make mm walkers work properly.
> +	 */
> +	if (mm_state) {
> +		spin_lock(&mm_list->lock);
> +		VM_WARN_ON_ONCE(mm_state->seq + 1 < max_seq);
> +		if (max_seq > mm_state->seq) {
> +			WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
> +			success = true;
> +		}
> +		spin_unlock(&mm_list->lock);
> +	} else {
> +		success = true;
> +	}
> +
> +	if (success)
> +		inc_max_seq(lruvec, max_seq, swappiness);
> +}
> +
> +/*
> + * We need to ensure that the folios of child memcg can be reparented to the
> + * same gen of the parent memcg, so the gens of the parent memcg needed be
> + * incremented to the MAX_NR_GENS before reparenting.
> + */
> +void max_lru_gen_memcg(struct mem_cgroup *memcg)
> +{
> +	int nid;
> +
> +	for_each_node(nid) {
> +		struct lruvec *lruvec = get_lruvec(memcg, nid);
> +		int type;
> +
> +		for (type = 0; type < ANON_AND_FILE; type++) {
> +			while (get_nr_gens(lruvec, type) < MAX_NR_GENS) {
> +				try_to_inc_max_seq_nowalk(memcg, lruvec);
> +				cond_resched();
> +			}
> +		}
> +	}
> +}
> +
> +/*
> + * Compared to traditional LRU, MGLRU faces the following challenges:
> + *
> + * 1. Each lruvec has between MIN_NR_GENS and MAX_NR_GENS generations, the
> + *    number of generations of the parent and child memcg may be different,
> + *    so we cannot simply transfer MGLRU folios in the child memcg to the
> + *    parent memcg as we did for traditional LRU folios.
> + * 2. The generation information is stored in folio->flags, but we cannot
> + *    traverse these folios while holding the lru lock, otherwise it may
> + *    cause softlockup.
> + * 3. In walk_update_folio(), the gen of folio and corresponding lru size
> + *    may be updated, but the folio is not immediately moved to the
> + *    corresponding lru list. Therefore, there may be folios of different
> + *    generations on an LRU list.
> + * 4. In lru_gen_del_folio(), the generation to which the folio belongs is
> + *    found based on the generation information in folio->flags, and the
> + *    corresponding LRU size will be updated. Therefore, we need to update
> + *    the lru size correctly during reparenting, otherwise the lru size may
> + *    be updated incorrectly in lru_gen_del_folio().
> + *
> + * Finally, we choose a compromise method, which is to splice the lru list in
> + * the child memcg to the lru list of the same generation in the parent memcg
> + * during reparenting.
> + *
> + * The same generation has different meanings in the parent and child memcg,
> + * so this compromise method will cause the LRU inversion problem. But as the
> + * system runs, this problem will be fixed automatically.
> + */
> +static void __lru_gen_reparent_memcg(struct lruvec *child_lruvec, struct lruvec *parent_lruvec,
> +				     int zone, int type)
> +{
> +	struct lru_gen_folio *child_lrugen, *parent_lrugen;
> +	enum lru_list lru = type * LRU_INACTIVE_FILE;
> +	int i;
> +
> +	child_lrugen = &child_lruvec->lrugen;
> +	parent_lrugen = &parent_lruvec->lrugen;
> +
> +	for (i = 0; i < get_nr_gens(child_lruvec, type); i++) {
> +		int gen = lru_gen_from_seq(child_lrugen->max_seq - i);
> +		long nr_pages = child_lrugen->nr_pages[gen][type][zone];
> +		int dst_lru_active = lru_gen_is_active(parent_lruvec, gen) ? LRU_ACTIVE : 0;
> +
> +		/* Assuming that child pages are colder than parent pages */
> +		list_splice_init(&child_lrugen->folios[gen][type][zone],
> +				 &parent_lrugen->folios[gen][type][zone]);
> +
> +		WRITE_ONCE(child_lrugen->nr_pages[gen][type][zone], 0);
> +		WRITE_ONCE(parent_lrugen->nr_pages[gen][type][zone],
> +			   parent_lrugen->nr_pages[gen][type][zone] + nr_pages);
> +
> +		update_lru_size(parent_lruvec, lru + dst_lru_active, zone, nr_pages);
> +	}
> +}
> +
> +void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent)
> +{
> +	int nid;
> +
> +	for_each_node(nid) {
> +		struct lruvec *child_lruvec, *parent_lruvec;
> +		int type, zid;
> +		struct zone *zone;
> +
> +		child_lruvec = get_lruvec(memcg, nid);
> +		parent_lruvec = get_lruvec(parent, nid);
> +
> +		for_each_managed_zone_pgdat(zone, NODE_DATA(nid), zid, MAX_NR_ZONES - 1) {
> +			for (type = 0; type < ANON_AND_FILE; type++)
> +				__lru_gen_reparent_memcg(child_lruvec, parent_lruvec, zid, type);
> +			mem_cgroup_update_lru_size(parent_lruvec, LRU_UNEVICTABLE, zid,
> +				mem_cgroup_get_zone_lru_size(child_lruvec, LRU_UNEVICTABLE, zid));
> +		}
> +	}
> +}
> +
>  #endif /* CONFIG_MEMCG */
>  
>  /******************************************************************************
> -- 
> 2.20.1
>