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

Harry Yoo
2025-11-26T13:49:09Z

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;
> +

I was testing this series and observed two warnings...

> + 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();

Warning 1) Here we increment max_seq but we skip updating mm_state->seq.
(try_to_inc_max_seq() iterates the mm list and update mm_state->seq after
an iteration, but since we directly call inc_max_seq(), we don't update it)

When mm_state->seq is more than one generation behind walk->seq, a warning is
triggered in iterate_mm_list():

VM_WARN_ON_ONCE(mm_state->seq + 1 < walk->max_seq);

Warning 2) In try_to_inc_max_seq(), the last walker of mm list
is supposed to succeed to increment max_seq by calling inc_max_seq():

if (success) {
success = inc_max_seq(lruvec, seq, swappiness);
WARN_ON_ONCE(!success);
}

But with this patch it may observe the max_seq is already advanced due to
reparenting and thus inc_max_seq() returns false, triggering the warning.

I'm learning MGLRU internals to see whether we can simply remove the warnings
or if we need to do something to advance max_seq without actually iterating
over the mm list.

--
Cheers,
Harry / Hyeonggon