Re: [PATCH v1 23/26] mm: vmscan: prepare for reparenting MGLRU folios

[Harry Yoo <harry.yoo@oracle.com>]

On Tue, Oct 28, 2025 at 09:58:36PM +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            | 86 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 102 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7aa8e1472d10d..3ee7fb96b8aeb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4468,6 +4468,92 @@ 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;
> +}
> +
> +/*
> + * 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) {
> +				DEFINE_MAX_SEQ(lruvec);
> +
> +				inc_max_seq(lruvec, max_seq, mem_cgroup_swappiness(memcg));
> +				cond_resched();
> +			}

To best of my knowledge this looks functionally correct.

> +		}
> +	}
> +}
> +
> +static void __lru_gen_reparent_memcg(struct lruvec *src_lruvec, struct lruvec *dst_lruvec,
> +				     int zone, int type)
> +{
> +	struct lru_gen_folio *src_lrugen, *dst_lrugen;
> +	enum lru_list lru = type * LRU_INACTIVE_FILE;
> +	int i;
> +
> +	src_lrugen = &src_lruvec->lrugen;
> +	dst_lrugen = &dst_lruvec->lrugen;
> +
> +	for (i = 0; i < get_nr_gens(src_lruvec, type); i++) {
> +		int gen = lru_gen_from_seq(src_lrugen->max_seq - i);
> +		int nr_pages = src_lrugen->nr_pages[gen][type][zone];

nr_pages should be long type since nothing prevents us from reparenting
more than 2 billions of pages :)

Otherwise looks correct to me.

-- 
Cheers,
Harry / Hyeonggon

> +		int src_lru_active = lru_gen_is_active(src_lruvec, gen) ? LRU_ACTIVE : 0;
> +		int dst_lru_active = lru_gen_is_active(dst_lruvec, gen) ? LRU_ACTIVE : 0;
> +
> +		list_splice_tail_init(&src_lrugen->folios[gen][type][zone],
> +				      &dst_lrugen->folios[gen][type][zone]);
> +
> +		WRITE_ONCE(src_lrugen->nr_pages[gen][type][zone], 0);
> +		WRITE_ONCE(dst_lrugen->nr_pages[gen][type][zone],
> +			   dst_lrugen->nr_pages[gen][type][zone] + nr_pages);
> +
> +		__update_lru_size(src_lruvec, lru + src_lru_active, zone, -nr_pages);
> +		__update_lru_size(dst_lruvec, lru + dst_lru_active, zone, nr_pages);
> +	}
> +}
> +
> +void lru_gen_reparent_memcg(struct mem_cgroup *src, struct mem_cgroup *dst)
> +{
> +	int nid;
> +
> +	for_each_node(nid) {
> +		struct lruvec *src_lruvec, *dst_lruvec;
> +		int type, zone;
> +
> +		src_lruvec = get_lruvec(src, nid);
> +		dst_lruvec = get_lruvec(dst, nid);
> +
> +		for (zone = 0; zone < MAX_NR_ZONES; zone++)
> +			for (type = 0; type < ANON_AND_FILE; type++)
> +				__lru_gen_reparent_memcg(src_lruvec, dst_lruvec, zone, type);
> +	}
> +}
> +
>  #endif /* CONFIG_MEMCG */
>  
>  /******************************************************************************
> -- 
> 2.20.1
>