linux-kernel - Re: [PATCH mm-unstable v1 2/4] mm/mglru: try to stop at high watermarks

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Date:   Mon, 11 Dec 2023 15:01:11 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Charan Teja Kalla <quic_charante@...cinc.com>,
        Jaroslav Pulchart <jaroslav.pulchart@...ddata.com>,
        Kalesh Singh <kaleshsingh@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH mm-unstable v1 2/4] mm/mglru: try to stop at high watermarks

On Fri, Dec 8, 2023 at 4:00 AM Hillf Danton <hdanton@...a.com> wrote:
>
> On Thu,  7 Dec 2023 23:14:05 -0700 Yu Zhao <yuzhao@...gle.com>
> > -static unsigned long get_nr_to_reclaim(struct scan_control *sc)
> > +static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
> >  {
> > +     int i;
> > +     enum zone_watermarks mark;
> > +
> >       /* don't abort memcg reclaim to ensure fairness */
> >       if (!root_reclaim(sc))
> > -             return -1;
> > +             return false;
> >
> > -     return max(sc->nr_to_reclaim, compact_gap(sc->order));
> > +     if (sc->nr_reclaimed >= max(sc->nr_to_reclaim, compact_gap(sc->order)))
> > +             return true;
> > +
> > +     /* check the order to exclude compaction-induced reclaim */
> > +     if (!current_is_kswapd() || sc->order)
> > +             return false;
> > +
> > +     mark = sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING ?
> > +            WMARK_PROMO : WMARK_HIGH;
> > +
> > +     for (i = 0; i <= sc->reclaim_idx; i++) {
> > +             struct zone *zone = lruvec_pgdat(lruvec)->node_zones + i;
> > +             unsigned long size = wmark_pages(zone, mark) + MIN_LRU_BATCH;
> > +
> > +             if (managed_zone(zone) && !zone_watermark_ok(zone, 0, size, sc->reclaim_idx, 0))
> > +                     return false;
> > +     }
> > +
> > +     /* kswapd should abort if all eligible zones are safe */
>
> This comment does not align with 86c79f6b5426
> ("mm: vmscan: do not reclaim from kswapd if there is any eligible zone").
> Any thing special here?

I don't see how they are not: they essentially say the same thing ("no
more than needed") but with different units: zones or pages. IOW,
don't reclaim from more zones or pages than needed.