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Message-ID: <kp4tov2zwphl23wpp5punfgh73st5grrlswsfmfk52yu45fz4d@22mwy742k2yq>
Date: Sat, 17 Jan 2026 19:25:32 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...nel.org,
lorenzo.stoakes@...cle.com, ziy@...dia.com, harry.yoo@...cle.com, yosry.ahmed@...ux.dev,
imran.f.khan@...cle.com, kamalesh.babulal@...cle.com, axelrasmussen@...gle.com,
yuanchu@...gle.com, weixugc@...gle.com, chenridong@...weicloud.com, mkoutny@...e.com,
akpm@...ux-foundation.org, hamzamahfooz@...ux.microsoft.com, apais@...ux.microsoft.com,
lance.yang@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
cgroups@...r.kernel.org, Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v3 26/30] mm: vmscan: prepare for reparenting MGLRU folios
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@...edance.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@...cle.com>
> Suggested-by: Imran Khan <imran.f.khan@...cle.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.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
>
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