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From:   Barry Song <21cnbao@gmail.com>
Date:   Sat, 19 Mar 2022 23:14:45 +1300
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Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation
To:     Yu Zhao <yuzhao@google.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>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Jesse Barnes <jsbarnes@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Michael Larabel <Michael@michaellarabel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Mike Rapoport <rppt@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
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        x86 <x86@kernel.org>, Brian Geffon <bgeffon@google.com>,
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        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Steven Barrett <steven@liquorix.net>,
        Suleiman Souhlal <suleiman@google.com>,
        Daniel Byrne <djbyrne@mtu.edu>,
        Donald Carr <d@chaos-reins.com>,
        =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= <holger@applied-asynchrony.com>,
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> +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][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?

isn't it a bug if(lrugen->nr_pages[next][type][zone] > 0)? shouldn't it be=
=EF=BC=9F

delta =3D lrugen->nr_pages[prev][type][zone]=EF=BC=9B

> +
> +                       if (!delta)
> +                               continue;
> +
> +                       __update_lru_size(lruvec, lru, zone, delta);
> +                       __update_lru_size(lruvec, lru + LRU_ACTIVE, zone,=
 -delta);
> +               }
> +       }
> +
> +       for (type =3D 0; type < ANON_AND_FILE; type++)
> +               reset_ctrl_pos(lruvec, type, false);
> +
> +       /* make sure preceding modifications appear */
> +       smp_store_release(&lrugen->max_seq, lrugen->max_seq + 1);
> +unlock:
> +       spin_unlock_irq(&lruvec->lru_lock);
> +}

Thanks
Barry