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

[Qi Zheng <qi.zheng@linux.dev>]

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@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);

The mm_state->seq is just to record the completion of a full traversal
of mm_list. If we simply delete this warning, it may cause this judgment
in iterate_mm_list to become invalid:

         if (walk->seq <= mm_state->seq)
		goto done;

So it seems we can manually increase mm_state->seq during reparenting to
avoid this warning.

However, we cannot directly call iterate_mm_list_nowalk() because we do
not want to reset mm_state->head and mm_state->tail to NULL. Otherwise,
we wouldn't be able to continue iterating over the mm_list.

> 
> 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.

After we correct the warning above, we will satisfy this condition in
iterate_mm_list():

         if (walk->seq <= mm_state->seq)
		goto done;

thus returning false, and avoiding triggering this 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.

So IIUC, we can simple increase mm_state->seq during reparenting to
fix these warnings.

>