Re: [PATCH 2/8] mm/mglru: relocate the LRU scan batch limit to callers

[Chen Ridong <chenridong@huaweicloud.com>]

On 2026/3/18 3:08, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> Same as active / inactive LRU, MGLRU isolates and scans folios in
> batches.  The batch split is done hidden deep in the helper, which
> makes the code harder to follow.  The helper's arguments are also
> confusing since callers usually request more folios than the batch
> size, so the helper almost never processes the full requested amount.
> 
> Move the batch splitting into the top loop to make it cleaner, there
> should be no behavior change.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>

I prefer to keep it as is.

If we move min(nr_to_scan, MAX_LRU_BATCH) out of scan_folios, callers
(potentially many functions in the future) would need to handle this logic
themselves, which seems unnecessary. The scan_folios helper should remain cohesive.

Thanks.

> ---
>  mm/vmscan.c | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d7fc7f1fe06d..d48074f9bd87 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4689,10 +4689,10 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	int scanned = 0;
>  	int isolated = 0;
>  	int skipped = 0;
> -	int scan_batch = min(nr_to_scan, MAX_LRU_BATCH);
> -	int remaining = scan_batch;
> +	unsigned long remaining = nr_to_scan;
>  	struct lru_gen_folio *lrugen = &lruvec->lrugen;
>  
> +	VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
>  	VM_WARN_ON_ONCE(!list_empty(list));
>  
>  	if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
> @@ -4745,7 +4745,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	mod_lruvec_state(lruvec, item, isolated);
>  	mod_lruvec_state(lruvec, PGREFILL, sorted);
>  	mod_lruvec_state(lruvec, PGSCAN_ANON + type, isolated);
> -	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, scan_batch,
> +	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
>  				scanned, skipped, isolated,
>  				type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>  	if (type == LRU_GEN_FILE)
> @@ -4827,7 +4827,8 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  
>  		*type_scanned = type;
>  
> -		scanned = scan_folios(nr_to_scan, lruvec, sc, type, tier, list);
> +		scanned = scan_folios(nr_to_scan, lruvec, sc,
> +				      type, tier, list);
>  		if (scanned)
>  			return scanned;
>  
> @@ -4999,7 +5000,7 @@ static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
>  
>  static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  {
> -	long nr_to_scan;
> +	long nr_batch, nr_to_scan;
>  	unsigned long scanned = 0;
>  	int swappiness = get_swappiness(lruvec, sc);
>  
> @@ -5010,7 +5011,8 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  		if (nr_to_scan <= 0)
>  			break;
>  
> -		delta = evict_folios(nr_to_scan, lruvec, sc, swappiness);
> +		nr_batch = min(nr_to_scan, MAX_LRU_BATCH);
> +		delta = evict_folios(nr_batch, lruvec, sc, swappiness);
>  		if (!delta)
>  			break;
>  
> @@ -5615,6 +5617,7 @@ static int run_aging(struct lruvec *lruvec, unsigned long seq,
>  static int run_eviction(struct lruvec *lruvec, unsigned long seq, struct scan_control *sc,
>  			int swappiness, unsigned long nr_to_reclaim)
>  {
> +	int nr_batch;
>  	DEFINE_MAX_SEQ(lruvec);
>  
>  	if (seq + MIN_NR_GENS > max_seq)
> @@ -5631,8 +5634,8 @@ static int run_eviction(struct lruvec *lruvec, unsigned long seq, struct scan_co
>  		if (sc->nr_reclaimed >= nr_to_reclaim)
>  			return 0;
>  
> -		if (!evict_folios(nr_to_reclaim - sc->nr_reclaimed, lruvec, sc,
> -				  swappiness))
> +		nr_batch = min(nr_to_reclaim - sc->nr_reclaimed, MAX_LRU_BATCH);
> +		if (!evict_folios(nr_batch, lruvec, sc, swappiness))
>  			return 0;
>  
>  		cond_resched();
> 

-- 
Best regards,
Ridong