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    SubjectRe: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation
    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 = &lruvec->lrugen;
    > > +
    > > + spin_lock_irq(&lruvec->lru_lock);
    > > +
    > > + VM_BUG_ON(!seq_is_valid(lruvec));
    > > +
    > > + if (max_seq != lrugen->max_seq)
    > > + goto unlock;
    > > +
    > > + inc_min_seq(lruvec);
    > > +
    > > + /* update the active/inactive LRU sizes for compatibility */
    > > + prev = lru_gen_from_seq(lrugen->max_seq - 1);
    > > + next = lru_gen_from_seq(lrugen->max_seq + 1);
    > > +
    > > + for (type = 0; type < ANON_AND_FILE; type++) {
    > > + for (zone = 0; zone < MAX_NR_ZONES; zone++) {
    > > + enum lru_list lru = type * LRU_INACTIVE_FILE;
    > > + long delta = lrugen->nr_pages[prev][type][zone] -
    > > + 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 be?
    >
    > delta = lrugen->nr_pages[prev][type][zone];

    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?

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