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

[Barry Song]

On Wed, Mar 18, 2026 at 3:11 AM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@xxxxxxxxxx> wrote:
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxx>
> ---
>  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);

Do we need to change this?

>                 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);

I wonder if we should modify get_nr_to_scan() to return
a maximum of 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);

Looks good to me.

> +               if (!evict_folios(nr_batch, lruvec, sc, swappiness))
>                         return 0;
>
>                 cond_resched();
>
> --
> 2.53.0

Thanks
Barry