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

Qi Zheng
2025-11-26T02:45:25Z

On 11/25/25 5:55 PM, Harry Yoo wrote:
> 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 :)

Right. The lru_gen_folio.nr_pages is long type, I don't know how I ended
up writing it as an int type.

Will fix it in the next version.

>
> Otherwise looks correct to me.

Thanks!

>