Re: [PATCH v2] mm/swapfile.c: introduce function alloc_swap_scan_list()

[Kairui Song <ryncsn@gmail.com>]

On Fri, Aug 8, 2025 at 3:48 PM Chris Li <chrisl@kernel.org> wrote:
>
> This the alloc_swap_scan_list() will scan the whole list or the first
> cluster.

Hi Chris,

This sentence reads strange to me, but English is not my native
language so I'm not very sure about it.

>
> This reduces the repeat patterns of isolating a cluster then scanning
> that cluster. As a result, cluster_alloc_swap_entry() is shorter and shallower.
>
> No functional change.
>
> Signed-off-by: Chris Li <chrisl@kernel.org>
> ---
> This patch goes on top of Kairui's swap improve cluster scan series:
> https://lore.kernel.org/linux-mm/20250806161748.76651-1-ryncsn@gmail.com/
> ---
> Changes in v2:
> - Adjust change base on Andrew's feedback on int type and break out of
>   loop.
> - Link to v1: https://lore.kernel.org/r/20250806-swap-scan-list-v1-1-a5fe2d9340a2@kernel.org
> ---
>  mm/swapfile.c | 86 ++++++++++++++++++++++++++++++++---------------------------
>  1 file changed, 47 insertions(+), 39 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4a0cf4fb348d..f26678d68874 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -820,6 +820,29 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
>         return found;
>  }
>
> +static unsigned int alloc_swap_scan_list(struct swap_info_struct *si,
> +                                        struct list_head *list,
> +                                        unsigned int order,
> +                                        unsigned char usage,
> +                                        bool scan_all)
> +{
> +       unsigned int found = SWAP_ENTRY_INVALID;
> +
> +       do {
> +               struct swap_cluster_info *ci = isolate_lock_cluster(si, list);
> +               unsigned long offset;
> +
> +               if (!ci)
> +                       break;
> +               offset = cluster_offset(si, ci);
> +               found = alloc_swap_scan_cluster(si, ci, offset, order, usage);
> +               if (found)
> +                       break;
> +       } while (scan_all);
> +
> +       return found;
> +}
> +
>  static void swap_reclaim_full_clusters(struct swap_info_struct *si, bool force)
>  {
>         long to_scan = 1;
> @@ -913,32 +936,24 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
>          * to spread out the writes.
>          */
>         if (si->flags & SWP_PAGE_DISCARD) {
> -               ci = isolate_lock_cluster(si, &si->free_clusters);
> -               if (ci) {
> -                       found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> -                                                       order, usage);
> -                       if (found)
> -                               goto done;
> -               }
> +               found = alloc_swap_scan_list(si, &si->free_clusters, order, usage,
> +                                            false);
> +               if (found)
> +                       goto done;
>         }
>
>         if (order < PMD_ORDER) {
> -               while ((ci = isolate_lock_cluster(si, &si->nonfull_clusters[order]))) {
> -                       found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> -                                                       order, usage);
> -                       if (found)
> -                               goto done;
> -               }
> +               found = alloc_swap_scan_list(si, &si->nonfull_clusters[order],
> +                                            order, usage, 0);

All other alloc_swap_scan_list calls use `false`/`true` but this one
uses `0` for `scan_all`.
And it should be `true`, right?

> +               if (found)
> +                       goto done;
>         }
>
>         if (!(si->flags & SWP_PAGE_DISCARD)) {
> -               ci = isolate_lock_cluster(si, &si->free_clusters);
> -               if (ci) {
> -                       found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> -                                                       order, usage);
> -                       if (found)
> -                               goto done;
> -               }
> +               found = alloc_swap_scan_list(si, &si->free_clusters, order, usage,
> +                                            false);
> +               if (found)
> +                       goto done;
>         }
>
>         /* Try reclaim full clusters if free and nonfull lists are drained */
> @@ -952,13 +967,10 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
>                  * failure is not critical. Scanning one cluster still
>                  * keeps the list rotated and reclaimed (for HAS_CACHE).
>                  */
> -               ci = isolate_lock_cluster(si, &si->frag_clusters[order]);
> -               if (ci) {
> -                       found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> -                                                       order, usage);
> -                       if (found)
> -                               goto done;
> -               }
> +               found = alloc_swap_scan_list(si, &si->frag_clusters[order], order,
> +                                            usage, true);

And it should be `false` here.

> +               if (found)
> +                       goto done;
>         }
>
>         /*
> @@ -977,19 +989,15 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
>                  * Clusters here have at least one usable slots and can't fail order 0
>                  * allocation, but reclaim may drop si->lock and race with another user.
>                  */
> -               while ((ci = isolate_lock_cluster(si, &si->frag_clusters[o]))) {
> -                       found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> -                                                       0, usage);
> -                       if (found)
> -                               goto done;
> -               }
> +               found = alloc_swap_scan_list(si, &si->frag_clusters[o],
> +                                            0, usage, true);
> +               if (found)
> +                       goto done;
>
> -               while ((ci = isolate_lock_cluster(si, &si->nonfull_clusters[o]))) {
> -                       found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
> -                                                       0, usage);
> -                       if (found)
> -                               goto done;
> -               }
> +               found = alloc_swap_scan_list(si, &si->nonfull_clusters[o],
> +                                            0, usage, true);
> +               if (found)
> +                       goto done;
>         }
>  done:
>         if (!(si->flags & SWP_SOLIDSTATE))
>
> ---
> base-commit: f89484324d5876ee10765fa61da0332899fa1a6a
> change-id: 20250806-swap-scan-list-2b89e3424b0a
>
> Best regards,
> --
> Chris Li <chrisl@kernel.org>
>

This makes the code much cleaner, thanks!