Re: [PATCH v3 1/9] mm/swapfile: add batched version of folio_dup_swap

[Kairui Song]

On Thu, Sep 24, 2026 at 9:15 PM Dev Jain <dev.jain@xxxxxxx> wrote:
>
> Add folio_dup_swap_pages to handle a batch of consecutive pages. Note
> that folio_dup_swap already can handle a subset of this: nr_pages == 1 and
> nr_pages == folio_nr_pages(folio). Generalize this to any nr_pages.
>
> Currently we have a not-so-nice logic of passing in subpage == NULL if
> we mean to exercise the logic on the entire folio, and subpage != NULL if
> we want to exercise the logic on only that subpage. Remove this
> indirection: the caller invokes folio_dup_swap_pages() if it wants to
> operate on a range of pages in the folio (i.e nr_pages may be anything
> between 1 and folio_nr_pages()), and invokes folio_dup_swap() if it
> wants to operate on the entire folio.
>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Reviewed-by: Barry Song <baohua@xxxxxxxxxx>
> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> ---
>  mm/rmap.c     |  2 +-
>  mm/shmem.c    |  2 +-
>  mm/swap.h     | 18 ++++++++++++++++--
>  mm/swapfile.c | 18 ++++++++----------
>  4 files changed, 26 insertions(+), 14 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 6661bc11ce658..a70dbc8d544e1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2150,7 +2150,7 @@ static bool ttu_anon_swapbacked_folio(struct vm_area_struct *vma,
>         swp_entry_t entry = folio_page_swap_entry(folio, page);
>         struct mm_struct *mm = vma->vm_mm;
>
> -       if (folio_dup_swap(folio, page) < 0)
> +       if (folio_dup_swap_pages(folio, page, 1) < 0)
>                 return false;
>
>         /*
> diff --git a/mm/shmem.c b/mm/shmem.c
> index b572c60f2af85..d5bd9506ce500 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1914,7 +1914,7 @@ int shmem_writeout(struct swap_io_ctx *ctx, struct folio *folio,
>                         spin_unlock(&shmem_swaplist_lock);
>                 }
>
> -               folio_dup_swap(folio, NULL);
> +               folio_dup_swap(folio);
>                 shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap));
>
>                 BUG_ON(folio_mapped(folio));
> diff --git a/mm/swap.h b/mm/swap.h
> index b3b54c28929a1..47290510de264 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -244,7 +244,8 @@ extern int swap_retry_table_alloc(swp_entry_t entry, gfp_t gfp);
>   * folio_put_swap(): does the opposite thing of folio_dup_swap().
>   */
>  int folio_alloc_swap(struct folio *folio);
> -int folio_dup_swap(struct folio *folio, struct page *page);
> +int folio_dup_swap_pages(struct folio *folio, struct page *page,
> +               unsigned long nr_pages);
>  void folio_put_swap(struct folio *folio, struct page *page);
>
>  /* For internal use */
> @@ -368,7 +369,8 @@ static inline int folio_alloc_swap(struct folio *folio)
>         return -EINVAL;
>  }
>
> -static inline int folio_dup_swap(struct folio *folio, struct page *page)
> +static inline int folio_dup_swap_pages(struct folio *folio, struct page *page,
> +               unsigned long nr_pages)
>  {
>         return -EINVAL;
>  }
> @@ -464,6 +466,18 @@ static inline void __swap_cache_replace_folio(struct swap_cluster_info *ci,
>  }
>  #endif /* CONFIG_SWAP */
>
> +/**
> + * folio_dup_swap() - Increase swap count of all swap entries of a folio.
> + * @folio: folio with swap entries bound.
> + *
> + * See folio_dup_swap_pages() for more information.
> + */
> +static inline int folio_dup_swap(struct folio *folio)
> +{
> +       return folio_dup_swap_pages(folio, folio_page(folio, 0),
> +                                   folio_nr_pages(folio));
> +}
> +
>  extern const struct swap_ops swap_bdev_ops;
>
>  int shmem_writeout(struct swap_io_ctx *ctx, struct folio *folio,
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index c1c5fbb3c909d..75167b8580cc0 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1799,9 +1799,10 @@ int folio_alloc_swap(struct folio *folio)
>  }
>
>  /**
> - * folio_dup_swap() - Increase swap count of swap entries of a folio.
> + * folio_dup_swap_pages() - Increase swap count of swap entries of a folio.
>   * @folio: folio with swap entries bounded.
> - * @page: if not NULL, only increase the swap count of this page.
> + * @page: the first page in the folio to increase the swap count for.
> + * @nr_pages: the number of pages in the folio to increase the swap count for.
>   *
>   * Typically called when the folio is unmapped and have its swap entry to
>   * take its place: Swap entries allocated to a folio has count == 0 and pinned
> @@ -1815,18 +1816,15 @@ int folio_alloc_swap(struct folio *folio)
>   * swap_put_entries_direct on its swap entry before this helper returns, or
>   * the swap count may underflow.
>   */
> -int folio_dup_swap(struct folio *folio, struct page *page)
> +int folio_dup_swap_pages(struct folio *folio, struct page *page,
> +               unsigned long nr_pages)
>  {
> -       swp_entry_t entry = folio->swap;
> -       unsigned long nr_pages = folio_nr_pages(folio);
> +       swp_entry_t entry = folio_page_swap_entry(folio, page);
> +       unsigned long idx = folio_page_idx(folio, page);
>
>         VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
>         VM_WARN_ON_FOLIO(!folio_test_swapcache(folio), folio);
> -
> -       if (page) {
> -               entry = folio_page_swap_entry(folio, page);
> -               nr_pages = 1;
> -       }
> +       VM_WARN_ON_FOLIO(idx + nr_pages > folio_nr_pages(folio), folio);
>
>         return swap_dup_entries_cluster(swap_entry_to_info(entry),
>                                         swp_offset(entry), nr_pages);
> --
> 2.43.0

Looks good to me.

Reviewed-by: Kairui Song <kasong@xxxxxxxxxxx>