Re: [PATCH -next 3/5] mm/mglru: extend shrink_one for both lrugen and non-lrugen

[Chen Ridong]

On 2025/12/16 5:13, Johannes Weiner wrote:
> On Tue, Dec 09, 2025 at 01:25:55AM +0000, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@xxxxxxxxxx>
>>
>> Currently, flush_reclaim_state is placed differently between
>> shrink_node_memcgs and shrink_many. shrink_many (only used for gen-LRU)
>> calls it after each lruvec is shrunk, while shrink_node_memcgs calls it
>> only after all lruvecs have been shrunk.
>>
>> This patch moves flush_reclaim_state into shrink_node_memcgs and calls it
>> after each lruvec. This unifies the behavior and is reasonable because:
>>
>> 1. flush_reclaim_state adds current->reclaim_state->reclaimed to
>>    sc->nr_reclaimed.
>> 2. For non-MGLRU root reclaim, this can help stop the iteration earlier
>>    when nr_to_reclaim is reached.
>> 3. For non-root reclaim, the effect is negligible since flush_reclaim_state
>>    does nothing in that case.
>>
>> After moving flush_reclaim_state into shrink_node_memcgs, shrink_one can be
>> extended to support both lrugen and non-lrugen paths. It will call
>> try_to_shrink_lruvec for lrugen root reclaim and shrink_lruvec otherwise.
>>
>> Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
>> ---
>>  mm/vmscan.c | 57 +++++++++++++++++++++--------------------------------
>>  1 file changed, 23 insertions(+), 34 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 584f41eb4c14..795f5ebd9341 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -4758,23 +4758,7 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>>  	return nr_to_scan < 0;
>>  }
>>  
>> -static void shrink_one(struct lruvec *lruvec, struct scan_control *sc)
>> -{
>> -	unsigned long scanned = sc->nr_scanned;
>> -	unsigned long reclaimed = sc->nr_reclaimed;
>> -	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>> -	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>> -
>> -	try_to_shrink_lruvec(lruvec, sc);
>> -
>> -	shrink_slab(sc->gfp_mask, pgdat->node_id, memcg, sc->priority);
>> -
>> -	if (!sc->proactive)
>> -		vmpressure(sc->gfp_mask, memcg, false, sc->nr_scanned - scanned,
>> -			   sc->nr_reclaimed - reclaimed);
>> -
>> -	flush_reclaim_state(sc);
>> -}
>> +static void shrink_one(struct lruvec *lruvec, struct scan_control *sc);
>>  
>>  static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)
>>  {
>> @@ -5760,6 +5744,27 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
>>  	return inactive_lru_pages > pages_for_compaction;
>>  }
>>  
>> +static void shrink_one(struct lruvec *lruvec, struct scan_control *sc)
>> +{
>> +	unsigned long scanned = sc->nr_scanned;
>> +	unsigned long reclaimed = sc->nr_reclaimed;
>> +	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>> +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>> +
>> +	if (lru_gen_enabled() && root_reclaim(sc))
>> +		try_to_shrink_lruvec(lruvec, sc);
>> +	else
>> +		shrink_lruvec(lruvec, sc);
> 

Hi Johannes, thank you for your reply.

> Yikes. So we end up with:
> 
> shrink_node_memcgs()
>   shrink_one()
>     if lru_gen_enabled && root_reclaim(sc)
>       try_to_shrink_lruvec(lruvec, sc)
>     else
>       shrink_lruvec()
>         if lru_gen_enabled && !root_reclaim(sc)
>           lru_gen_shrink_lruvec(lruvec, sc)
>             try_to_shrink_lruvec()
> 
> I think it's doing too much at once. Can you get it into the following
> shape:
> 

You're absolutely right. This refactoring is indeed what patch 5/5 implements.

With patch 5/5 applied, the flow becomes:

shrink_node_memcgs()
    shrink_one()
        if lru_gen_enabled
	    lru_gen_shrink_lruvec  --> symmetric with else shrink_lruvec()
		if (root_reclaim(sc))  --> handle root reclaim.
		    try_to_shrink_lruvec()
		else
		    ...
		    try_to_shrink_lruvec()
	else
	    shrink_lruvec()

This matches the structure you described.

One note: shrink_one() is also called from lru_gen_shrink_node() when memcg is disabled, so I
believe it makes sense to keep this helper.

> shrink_node_memcgs()
>   for each memcg:
>     if lru_gen_enabled:
>       lru_gen_shrink_lruvec()
>     else
>       shrink_lruvec()
> 

Regarding the patch split, I currently kept patch 3/5 and 5/5 separate to make the changes clearer
in each step. Would you prefer that I merge patch 3/5 with patch 5/5, so the full refactoring
appears in one patch?

Looking forward to your guidance.

> and handle the differences in those two functions? Then look for
> overlap one level down, and so forth.

-- 
Best regards,
Ridong