Re: [PATCH v5 05/14] mm/mglru: scan and count the exact number of folios

[Kairui Song <ryncsn@gmail.com>]

On Thu, Apr 16, 2026 at 3:03 PM Barry Song <baohua@kernel.org> wrote:
>
> On Mon, Apr 13, 2026 at 12:48 AM Kairui Song via B4 Relay
> <devnull+kasong.tencent.com@kernel.org> wrote:
> >
> >  static bool inc_max_seq(struct lruvec *lruvec, unsigned long seq, int swappiness)
> > @@ -4686,7 +4681,7 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
> >
> >  static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
> >                        struct scan_control *sc, int type, int tier,
> > -                      struct list_head *list)
> > +                      struct list_head *list, int *isolatedp)
> >  {
> >         int i;
> >         int gen;
> > @@ -4756,11 +4751,9 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
> >                                 type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
> >         if (type == LRU_GEN_FILE)
> >                 sc->nr.file_taken += isolated;
> > -       /*
> > -        * There might not be eligible folios due to reclaim_idx. Check the
> > -        * remaining to prevent livelock if it's not making progress.
> > -        */
> > -       return isolated || !remaining ? scanned : 0;
> > +
> > +       *isolatedp = isolated;
> > +       return scanned;
> >  }
> >
> >  static int get_tier_idx(struct lruvec *lruvec, int type)
> > @@ -4804,33 +4797,36 @@ static int get_type_to_scan(struct lruvec *lruvec, int swappiness)
> >
> >  static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
> >                           struct scan_control *sc, int swappiness,
> > -                         int *type_scanned, struct list_head *list)
> > +                         struct list_head *list, int *isolated,
> > +                         int *isolate_type, int *isolate_scanned)
> >  {
> >         int i;
> > +       int scanned = 0;
>
> I would prefer to rename this to total_scanned.

Good suggestion.

>
> >         int type = get_type_to_scan(lruvec, swappiness);
> >
> >         for_each_evictable_type(i, swappiness) {
> > -               int scanned;
> > +               int type_scan;
>
> And then we keep this as "scanned".
>
> >                 int tier = get_tier_idx(lruvec, type);
> >
> > -               *type_scanned = type;
> > +               type_scan = scan_folios(nr_to_scan, lruvec, sc,
> > +                                       type, tier, list, isolated);
> >
> > -               scanned = scan_folios(nr_to_scan, lruvec, sc, type, tier, list);
> > -               if (scanned)
> > -                       return scanned;
> > +               scanned += type_scan;
> > +               if (*isolated) {
> > +                       *isolate_type = type;
> > +                       *isolate_scanned = type_scan;
> > +                       break;
> > +               }
> >
> >                 type = !type;
> >         }
> >
> > -       return 0;
> > +       return scanned;
>
> Then
> return total_scanned;
>
> >  }
> >
> >  static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
> >                         struct scan_control *sc, int swappiness)
> >  {
> > -       int type;
> > -       int scanned;
> > -       int reclaimed;
> >         LIST_HEAD(list);
> >         LIST_HEAD(clean);
> >         struct folio *folio;
> > @@ -4838,19 +4834,23 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
> >         enum node_stat_item item;
> >         struct reclaim_stat stat;
> >         struct lru_gen_mm_walk *walk;
> > +       int scanned, reclaimed;
> > +       int isolated = 0, type, type_scanned;
> >         bool skip_retry = false;
> > -       struct lru_gen_folio *lrugen = &lruvec->lrugen;
> >         struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> >         struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> >
> >         lruvec_lock_irq(lruvec);
> >
> > -       scanned = isolate_folios(nr_to_scan, lruvec, sc, swappiness, &type, &list);
> > +       /* In case folio deletion left empty old gens, flush them */
> > +       try_to_inc_min_seq(lruvec, swappiness);
> >
> > -       scanned += try_to_inc_min_seq(lruvec, swappiness);
> > +       scanned = isolate_folios(nr_to_scan, lruvec, sc, swappiness,
> > +                                &list, &isolated, &type, &type_scanned);
> >
> > -       if (evictable_min_seq(lrugen->min_seq, swappiness) + MIN_NR_GENS > lrugen->max_seq)
> > -               scanned = 0;
> > +       /* Isolation might create empty gen, flush them */
> > +       if (scanned)
>
> scanned is not equal to isolated, right?
> Somehow, I feel the comment does not match the if (scanned).
> I assume sort_folio() could also create empty gen?

Yeah, you are right, it should be isolated. Usually one batch scan
always isolates a few folios so I didn't observe much difference in
testing, but I should definitely fix this. Thanks!