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From:   Yu Zhao <yuzhao@google.com>
Date:   Tue, 22 Mar 2022 02:20:20 -0600
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Subject: Re: [PATCH v9 10/14] mm: multi-gen LRU: kill switch
To:     Barry Song <21cnbao@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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On Tue, Mar 22, 2022 at 1:47 AM Barry Song <21cnbao@gmail.com> wrote:
>
...
> > +static bool drain_evictable(struct lruvec *lruvec)
> > +{
> > +       int gen, type, zone;
> > +       int remaining = MAX_LRU_BATCH;
> > +
> > +       for_each_gen_type_zone(gen, type, zone) {
> > +               struct list_head *head = &lruvec->lrugen.lists[gen][type][zone];
> > +
> > +               while (!list_empty(head)) {
> > +                       bool success;
> > +                       struct folio *folio = lru_to_folio(head);
> > +
> > +                       VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
> > +                       VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
> > +                       VM_BUG_ON_FOLIO(folio_is_file_lru(folio) != type, folio);
> > +                       VM_BUG_ON_FOLIO(folio_zonenum(folio) != zone, folio);
> > +
> > +                       success = lru_gen_del_folio(lruvec, folio, false);
> > +                       VM_BUG_ON(!success);
> > +                       lruvec_add_folio(lruvec, folio);
>
> for example, max_seq=4(GEN=0) and max_seq-1=3, then we are supposed to put
> max_seq in the head of active list. but your code seems to be putting max_seq-1
> after putting max_seq, then max_seq is more likely to be evicted
> afterwards as it
> is in the tail of the active list.

This is correct.

> anyway, it might not be so important. I can't imagine we will
> frequently switch mglru
> with lru dynamically. will we?

I certainly hope not :)