linux-kernel - Re: [PATCH] vm_swappiness=0 should still try to avoid swapping anon memory

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Message-ID: <CALvZod6gCof1bhVwdU7vYYKBRCn_HZBFi4BjSYoSK-dyrmswMA@mail.gmail.com>
Date:   Fri, 6 Aug 2021 18:00:51 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Nico Pache <npache@...hat.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH] vm_swappiness=0 should still try to avoid swapping anon memory

On Fri, Aug 6, 2021 at 4:17 PM Nico Pache <npache@...hat.com> wrote:
>
> Since commit b91ac374346b ("mm: vmscan: enforce inactive:active ratio at the
> reclaim root") swappiness can start prematurely swapping anon memory.
> This is due to the assumption that refaulting anon should always allow
> the shrinker to target anon memory. Add a check for vm_swappiness being
> >0 before indiscriminately targeting Anon.

Did you actually observe this behavior?

>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  mm/vmscan.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 4620df62f0ff..8b932ff72e37 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2909,8 +2909,8 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>
>                 refaults = lruvec_page_state(target_lruvec,
>                                 WORKINGSET_ACTIVATE_ANON);
> -               if (refaults != target_lruvec->refaults[0] ||
> -                       inactive_is_low(target_lruvec, LRU_INACTIVE_ANON))
> +               if (vm_swappiness && (refaults != target_lruvec->refaults[0] ||
> +                       inactive_is_low(target_lruvec, LRU_INACTIVE_ANON)))

If you are really seeing the said behavior then why will this fix it.
This is just about deactivating active anon LRU. I would rather look
at get_scan_count() to check why swappiness = 0 is still letting the
kernel to scan anon LRU. BTW in cgroup v1, the memcg can overwrite
their swappiness which will be preferred over system vm_swappiness.
Did you set system level swappiness or memcg one?

>                         sc->may_deactivate |= DEACTIVATE_ANON;
>                 else
>                         sc->may_deactivate &= ~DEACTIVATE_ANON;
> --
> 2.31.1
>