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From:   Yu Zhao <yuzhao@google.com>
Date:   Mon, 21 Mar 2022 17:51:39 -0600
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Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation
To:     Barry Song <21cnbao@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Aneesh Kumar <aneesh.kumar@linux.ibm.com>,
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On Sat, Mar 19, 2022 at 4:14 AM Barry Song <21cnbao@gmail.com> wrote:
>
> > +static void inc_max_seq(struct lruvec *lruvec, unsigned long max_seq)
> > +{
> > +       int prev, next;
> > +       int type, zone;
> > +       struct lru_gen_struct *lrugen =3D &lruvec->lrugen;
> > +
> > +       spin_lock_irq(&lruvec->lru_lock);
> > +
> > +       VM_BUG_ON(!seq_is_valid(lruvec));
> > +
> > +       if (max_seq !=3D lrugen->max_seq)
> > +               goto unlock;
> > +
> > +       inc_min_seq(lruvec);
> > +
> > +       /* update the active/inactive LRU sizes for compatibility */
> > +       prev =3D lru_gen_from_seq(lrugen->max_seq - 1);
> > +       next =3D lru_gen_from_seq(lrugen->max_seq + 1);
> > +
> > +       for (type =3D 0; type < ANON_AND_FILE; type++) {
> > +               for (zone =3D 0; zone < MAX_NR_ZONES; zone++) {
> > +                       enum lru_list lru =3D type * LRU_INACTIVE_FILE;
> > +                       long delta =3D lrugen->nr_pages[prev][type][zon=
e] -
> > +                                    lrugen->nr_pages[next][type][zone]=
;
>
> this is confusing to me. does lrugen->nr_pages[next][type][zone] have a
> chance to be none-zero even before max_seq is increased? some pages
> can be in the next generation before the generation is born?

Yes.

> isn't it a bug if(lrugen->nr_pages[next][type][zone] > 0)? shouldn't it b=
e=EF=BC=9F
>
> delta =3D lrugen->nr_pages[prev][type][zone]=EF=BC=9B

No. The gen counter in page flags can be updated locklessly
(lru_lock). Later a batched update of nr_pages[] will account for the
change made. If the gen counter is updated to a stale max_seq, and
this stale max_seq is less than min_seq, then this page will be in a
generation yet to be born. Extremely unlikely, but still possible.

This is not a bug because pages might be misplaced but they won't be
lost. IOW, nr_pages[] is always balanced across all *possible*
generations. For the same reason, reset_batch_size() and
drain_evictable() use for_each_gen_type_zone() to go through all
possible generations rather than only those between[max_seq, min_seq].

I'll add a comment here. Sounds good?