Re: [PATCH v2 1/8] mm/swap: add folio_swap_entry() and folio_page_swap_entry()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/8/26 17:16, Tal Zussman wrote:
> A folio in the swap cache occupies folio_nr_pages() contiguous swap
> entries starting at folio->swap, so a page's swap entry is just
> folio->swap plus the page's index in the folio. page_swap_entry() hides
> this behind a compound_head() call, and callers that already have the
> folio sometimes open-code the arithmetic instead.
> 
> Add folio_swap_entry(), which takes a folio and a page index, and
> folio_page_swap_entry() for callers that have the page.
> 
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> ---
>  include/linux/swap.h | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index fc290e29e4a9..1d979e76e78a 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -272,6 +272,39 @@ struct swap_info_struct {
>  	const struct swap_ops *ops;
>  };
>  
> +/**
> + * folio_swap_entry - Return the swap entry for a page within a folio.
> + * @folio: The folio.
> + * @idx: The index of the page within the folio.
> + *
> + * A folio in the swap cache occupies folio_nr_pages() contiguous swap
> + * entries starting at folio->swap. The caller must ensure the folio is
> + * in the swap cache and that @idx is within the folio.
> + */
> +static inline
> +swp_entry_t folio_swap_entry(const struct folio *folio, unsigned long idx)
> +{
> +	swp_entry_t entry = folio->swap;
> +
> +	VM_WARN_ON_ONCE_FOLIO(idx >= folio_nr_pages(folio), folio);
> +	entry.val += idx;
> +	return entry;
> +}
> +
> +/**
> + * folio_page_swap_entry - Return the swap entry of a page in a folio.
> + * @folio: The folio containing @page.
> + * @page: A page within @folio.
> + *
> + * The caller must ensure the folio is in the swap cache and that @page
> + * is part of @folio.
> + */
> +static inline swp_entry_t folio_page_swap_entry(const struct folio *folio,
> +		const struct page *page)
> +{
> +	return folio_swap_entry(folio, folio_page_idx(folio, page));
> +}
> +
>  static inline swp_entry_t page_swap_entry(struct page *page)
>  {
>  	struct folio *folio = page_folio(page);
> 

"Return the swap entry for a page within a folio" vs. "Return the swap entry of
a page in a folio."

yet only of the variants has a "page" in the name :)

A bit confusing.

Not immediately sure how it could be done cleaner. The minority of cases seem to
use folio_swap_entry.

-- 
Cheers,

David