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

[Kairui Song]

On Sun, Mar 22, 2026 at 04:14:31PM +0800, Barry Song wrote:
> 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?

That's a irrelevant blank line change, will drop it, thanks!
> 
> >                 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？

We'll change that in a later commit to let each iteration use a smaller batch.