Re: [PATCH v3 12/12] mm, swap: no need to clear the shadow explicitly

[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>
>
> Since we no longer bypass the swap cache, every swap-in will clear the
> swap shadow by inserting the folio into the swap table. The only place
> we may seem to need to free the swap shadow is when the swap slots are
> freed directly without a folio (swap_put_entries_direct). But with the
> swap table, that is not needed either. Freeing a slot in the swap table
> will set the table entry to NULL, which erases the shadow just fine.
>
> So just delete all explicit shadow clearing, it's no longer needed.
> Also, rearrange the freeing.
>
> Signed-off-by: Kairui Song <kasong@tencent.com>

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

Chris

> ---
>  mm/swap.h       |  1 -
>  mm/swap_state.c | 21 ---------------------
>  mm/swapfile.c   |  2 --
>  3 files changed, 24 deletions(-)
>
> diff --git a/mm/swap.h b/mm/swap.h
> index 9728e6a944b2..a77016f2423b 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -290,7 +290,6 @@ void __swap_cache_del_folio(struct swap_cluster_info *ci,
>                             struct folio *folio, swp_entry_t entry, void *shadow);
>  void __swap_cache_replace_folio(struct swap_cluster_info *ci,
>                                 struct folio *old, struct folio *new);
> -void __swap_cache_clear_shadow(swp_entry_t entry, int nr_ents);
>
>  void show_swap_cache_info(void);
>  void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr);
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index e7618ffe6d70..32d9d877bda8 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -350,27 +350,6 @@ void __swap_cache_replace_folio(struct swap_cluster_info *ci,
>         }
>  }
>
> -/**
> - * __swap_cache_clear_shadow - Clears a set of shadows in the swap cache.
> - * @entry: The starting index entry.
> - * @nr_ents: How many slots need to be cleared.
> - *
> - * Context: Caller must ensure the range is valid, all in one single cluster,
> - * not occupied by any folio, and lock the cluster.
> - */
> -void __swap_cache_clear_shadow(swp_entry_t entry, int nr_ents)
> -{
> -       struct swap_cluster_info *ci = __swap_entry_to_cluster(entry);
> -       unsigned int ci_off = swp_cluster_offset(entry), ci_end;
> -       unsigned long old;
> -
> -       ci_end = ci_off + nr_ents;
> -       do {
> -               old = __swap_table_xchg(ci, ci_off, null_to_swp_tb());
> -               WARN_ON_ONCE(swp_tb_is_folio(old) || swp_tb_get_count(old));
> -       } while (++ci_off < ci_end);
> -}
> -
>  /*
>   * If we are the only user, then try to free up the swap cache.
>   *
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index dab5e726855b..802efa37b33f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1287,7 +1287,6 @@ static void swap_range_alloc(struct swap_info_struct *si,
>  static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
>                             unsigned int nr_entries)
>  {
> -       unsigned long begin = offset;
>         unsigned long end = offset + nr_entries - 1;
>         void (*swap_slot_free_notify)(struct block_device *, unsigned long);
>         unsigned int i;
> @@ -1312,7 +1311,6 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
>                         swap_slot_free_notify(si->bdev, offset);
>                 offset++;
>         }
> -       __swap_cache_clear_shadow(swp_entry(si->type, begin), nr_entries);
>
>         /*
>          * Make sure that try_to_unuse() observes si->inuse_pages reaching 0
>
> --
> 2.52.0
>
>