linux-kernel - Re: [PATCH] mm/vmscan: consider previously reclaimed pages in shrink

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Date:   Wed, 8 Nov 2023 19:25:20 +0900
From:   Hyeongtak Ji <hyeongtak.ji@...il.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hyeongtak Ji <hyeongtak.ji@...com>
Subject: Re: [PATCH] mm/vmscan: consider previously reclaimed pages in shrink_lruvec()

Hello,

Thank you for your reply.

On Wed, Nov 8, 2023 at 3:33 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Mon, Aug 07, 2023 at 07:01:16PM +0900, Hyeongtak Ji wrote:
> > shrink_lruvec() currently ignores previously reclaimed pages in
> > scan_control->nr_reclaimed.  This can lead shrink_lruvec() to reclaiming
> > more pages than expected.
> >
> > This patch fixes shrink_lruvec() to take into account the previously
> > reclaimed pages.
>
> Do you run into real world issues from this? The code has been like
> this for at least a decade.
>

I believed this was merely a misinitialization that resulted in
shrink_lruvec() reclaiming more pages than intended. However, I do
acknowledge that there have not been any real world issues arising from
this behavior.

> It's an intentional choice to ensure fairness across all visited
> cgroups. sc->nr_to_reclaim is 32 pages or less - it's only to guard

sc->nr_to_reclaim can be larger than 32 (e.g., about 5K) in the case that I
was worrying about. kswapd_shrink_node() in mm/vmscan.c sets the value
and it is passed down to shrink_lruvec().

> against extreme overreclaim. But we want to make sure we reclaim a bit
> from all cgroups, rather than always hit the first one and then bail.