linux-kernel - Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

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Message-ID: <CAGsJ_4wW-SUPZ+soz5L+TROcmWtiYoABMcYzNyw4p=f2ro9vLQ@mail.gmail.com>
Date:   Sat, 19 Mar 2022 23:14:45 +1300
From:   Barry Song <21cnbao@...il.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Kernel Page Reclaim v2 <page-reclaim@...gle.com>,
        x86 <x86@...nel.org>, Brian Geffon <bgeffon@...gle.com>,
        Jan Alexander Steffens <heftig@...hlinux.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Steven Barrett <steven@...uorix.net>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Daniel Byrne <djbyrne@....edu>,
        Donald Carr <d@...os-reins.com>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

> +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?

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

delta = lrugen->nr_pages[prev][type][zone]；

> +
> +                       if (!delta)
> +                               continue;
> +
> +                       __update_lru_size(lruvec, lru, zone, delta);
> +                       __update_lru_size(lruvec, lru + LRU_ACTIVE, zone, -delta);
> +               }
> +       }
> +
> +       for (type = 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