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

[Qi Zheng]

On 11/26/25 9:48 PM, Harry Yoo wrote:
> On Tue, Oct 28, 2025 at 09:58:36PM +0800, Qi Zheng wrote:
> > From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Suggested-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > ---
> >   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.

Got it. Thanks for testing and reporting!

> 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.

Thanks! I will also check on this.

>