Re: [PATCH v3 04/12] mm, swap: consolidate bad slots setup and make it more robust

[Chris Li <chrisl@kernel.org>]

On Tue, Feb 17, 2026 at 12:06 PM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@kernel.org> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> In preparation for using the swap table to track bad slots directly,
> move the bad slot setup to one place, set up the swap_map mark, and
> cluster counter update together.
>
> While at it, provide more informative logs and a more robust fallback if
> any bad slot info looks incorrect.
>
> Fixes a potential issue that a malformed swap file may cause the cluster
> to be unusable upon swapon, and provides a more verbose warning on a
> malformed swap file
>
> Signed-off-by: Kairui Song <kasong@tencent.com>

Acked-by: Chris Li <chrisl@kernel.org>

Chris

> ---
>  mm/swapfile.c | 68 +++++++++++++++++++++++++++++++++--------------------------
>  1 file changed, 38 insertions(+), 30 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index fb0d48681c48..91c1fa804185 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -743,13 +743,37 @@ static void relocate_cluster(struct swap_info_struct *si,
>   * slot. The cluster will not be added to the free cluster list, and its
>   * usage counter will be increased by 1. Only used for initialization.
>   */
> -static int swap_cluster_setup_bad_slot(struct swap_cluster_info *cluster_info,
> -                                      unsigned long offset)
> +static int swap_cluster_setup_bad_slot(struct swap_info_struct *si,
> +                                      struct swap_cluster_info *cluster_info,
> +                                      unsigned int offset, bool mask)
>  {
>         unsigned long idx = offset / SWAPFILE_CLUSTER;
>         struct swap_table *table;
>         struct swap_cluster_info *ci;
>
> +       /* si->max may got shrunk by swap swap_activate() */
> +       if (offset >= si->max && !mask) {
> +               pr_debug("Ignoring bad slot %u (max: %u)\n", offset, si->max);
> +               return 0;
> +       }
> +       /*
> +        * Account it, skip header slot: si->pages is initiated as
> +        * si->max - 1. Also skip the masking of last cluster,
> +        * si->pages doesn't include that part.
> +        */
> +       if (offset && !mask)
> +               si->pages -= 1;
> +       if (!si->pages) {
> +               pr_warn("Empty swap-file\n");
> +               return -EINVAL;
> +       }
> +       /* Check for duplicated bad swap slots. */
> +       if (si->swap_map[offset]) {
> +               pr_warn("Duplicated bad slot offset %d\n", offset);
> +               return -EINVAL;
> +       }
> +
> +       si->swap_map[offset] = SWAP_MAP_BAD;
>         ci = cluster_info + idx;
>         if (!ci->table) {
>                 table = swap_table_alloc(GFP_KERNEL);
> @@ -3227,30 +3251,12 @@ static int setup_swap_map(struct swap_info_struct *si,
>                           union swap_header *swap_header,
>                           unsigned long maxpages)
>  {
> -       unsigned long i;
>         unsigned char *swap_map;
>
>         swap_map = vzalloc(maxpages);
>         si->swap_map = swap_map;
>         if (!swap_map)
>                 return -ENOMEM;
> -
> -       swap_map[0] = SWAP_MAP_BAD; /* omit header page */
> -       for (i = 0; i < swap_header->info.nr_badpages; i++) {
> -               unsigned int page_nr = swap_header->info.badpages[i];
> -               if (page_nr == 0 || page_nr > swap_header->info.last_page)
> -                       return -EINVAL;
> -               if (page_nr < maxpages) {
> -                       swap_map[page_nr] = SWAP_MAP_BAD;
> -                       si->pages--;
> -               }
> -       }
> -
> -       if (!si->pages) {
> -               pr_warn("Empty swap-file\n");
> -               return -EINVAL;
> -       }
> -
>         return 0;
>  }
>
> @@ -3281,26 +3287,28 @@ static int setup_swap_clusters_info(struct swap_info_struct *si,
>         }
>
>         /*
> -        * Mark unusable pages as unavailable. The clusters aren't
> -        * marked free yet, so no list operations are involved yet.
> -        *
> -        * See setup_swap_map(): header page, bad pages,
> -        * and the EOF part of the last cluster.
> +        * Mark unusable pages (header page, bad pages, and the EOF part of
> +        * the last cluster) as unavailable. The clusters aren't marked free
> +        * yet, so no list operations are involved yet.
>          */
> -       err = swap_cluster_setup_bad_slot(cluster_info, 0);
> +       err = swap_cluster_setup_bad_slot(si, cluster_info, 0, false);
>         if (err)
>                 goto err;
>         for (i = 0; i < swap_header->info.nr_badpages; i++) {
>                 unsigned int page_nr = swap_header->info.badpages[i];
>
> -               if (page_nr >= maxpages)
> -                       continue;
> -               err = swap_cluster_setup_bad_slot(cluster_info, page_nr);
> +               if (!page_nr || page_nr > swap_header->info.last_page) {
> +                       pr_warn("Bad slot offset is out of border: %d (last_page: %d)\n",
> +                               page_nr, swap_header->info.last_page);
> +                       err = -EINVAL;
> +                       goto err;
> +               }
> +               err = swap_cluster_setup_bad_slot(si, cluster_info, page_nr, false);
>                 if (err)
>                         goto err;
>         }
>         for (i = maxpages; i < round_up(maxpages, SWAPFILE_CLUSTER); i++) {
> -               err = swap_cluster_setup_bad_slot(cluster_info, i);
> +               err = swap_cluster_setup_bad_slot(si, cluster_info, i, true);
>                 if (err)
>                         goto err;
>         }
>
> --
> 2.52.0
>
>