linux-kernel - Re: [PATCH 04/12] mm/swap: skip scanning cluster range if it's empty cluster

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Message-ID: <CAMgjq7CYC4xfAkp9Y7g=3rw=iqTj_pQAjrnwDC1orkdUhnOCSQ@mail.gmail.com>
Date: Thu, 6 Feb 2025 01:07:14 +0800
From: Kairui Song <ryncsn@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	akpm@...ux-foundation.org, chrisl@...nel.org
Subject: Re: [PATCH 04/12] mm/swap: skip scanning cluster range if it's empty cluster

On Wed, Feb 5, 2025 at 5:27 PM Baoquan He <bhe@...hat.com> wrote:
>
> Since ci->lock has been taken when isolating cluster from
> si->free_clusters or taking si->percpu_cluster->next[order],
> it's unnecessary to scan and check the cluster range availability
> if i'ts empty cluster, and this can accelerate the huge page
> swapping.
>
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  mm/swapfile.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 9c9a4ec6d4c6..61efde853eea 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -729,6 +729,9 @@ static bool cluster_scan_range(struct swap_info_struct *si,
>         unsigned long offset, end = start + nr_pages;
>         unsigned char *map = si->swap_map;
>
> +       if (cluster_is_empty(ci))
> +               return true;
> +

Hi Baoquan,

Thanks for the series.

Most commits are looking great, but this one is a bit questionable.
cluster_scan_range is only called by alloc_swap_scan_cluster, and it
already checks if the cluster has enough empty slots to use, so this
might be redundant.

It is possible that cluster_scan_range sees an empty cluster if the
cluster lock was dropped for reclaiming HAS_CACHE, but the chance
should be extremely low, that this might be a negative optimization.