linux-kernel - Re: [RFC PATCH v1 3/4] mm: Fix demotion-only scanning anon pages

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Date:   Thu, 24 Nov 2022 13:27:51 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>, weixugc@...gle.com,
        shakeelb@...gle.com, gthelen@...gle.com, fvdl@...gle.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <songmuchun@...edance.com>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 3/4] mm: Fix demotion-only scanning anon pages

Mina Almasry <almasrymina@...gle.com> writes:

> This is likely a missed change from commit a2a36488a61c ("mm/vmscan: Consider
> anonymous pages without swap"). Current logic is if !may_swap _or_
> !can_reclaim_anon_pages() then we don't scan anon memory.
>
> This should be an 'and'. We would like to scan anon memory if we may swap
> or if we can_reclaim_anon_pages().
>
> Fixes: commit a2a36488a61c ("mm/vmscan: Consider anonymous pages without swap")
>
> Signed-off-by: Mina Almasry <almasrymina@...gle.com>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8c1f5416d789..d7e509b3f07f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2931,7 +2931,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  	enum lru_list lru;
>
>  	/* If we have no swap space, do not bother scanning anon folios. */
> -	if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
> +	if (!sc->may_swap && !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {

If there is only DRAM but swap device is enabled, sc->may_swap will not
work as expected.  So we should check can_demote() instead if
sc->may_swap.

Best Regards,
Huang, Ying

>  		scan_balance = SCAN_FILE;
>  		goto out;
>  	}
> --
> 2.38.1.584.g0f3c55d4c2-goog