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

Chen Ridong
2026-03-19T02:00:25Z

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