Re: [PATCH v3 6/9] mm/swapfile: Add batched version of folio_dup_swap

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

On 5/6/26 11:45, Dev Jain 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 till folio_nr_pages()), and invokes folio_dup_swap() if it
> wants to operate on the entire folio.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/rmap.c     |  2 +-
>  mm/shmem.c    |  2 +-
>  mm/swap.h     | 12 ++++++++++--
>  mm/swapfile.c | 20 ++++++++++++--------
>  4 files changed, 24 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 25813e3605991..352ba77d90f67 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2314,7 +2314,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  				goto finish_unmap;
>  			}
>  
> -			if (folio_dup_swap(folio, subpage) < 0) {
> +			if (folio_dup_swap_pages(folio, subpage, 1) < 0) {

Can you throw in a patch to rename subpage -> page first?

>  				set_pte_at(mm, address, pvmw.pte, pteval);
>  				goto walk_abort;
>  			}
> diff --git a/mm/shmem.c b/mm/shmem.c
> index bab3529af23c5..5e4f521399847 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1698,7 +1698,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>  			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 a77016f2423b9..3c25f914e908b 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -206,7 +206,9 @@ 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 *subpage);
> +int folio_dup_swap(struct folio *folio);
> +int folio_dup_swap_pages(struct folio *folio, struct page *page,
> +			 unsigned long nr_pages);
>  void folio_put_swap(struct folio *folio, struct page *subpage);
>  
>  /* For internal use */
> @@ -390,7 +392,13 @@ 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(struct folio *folio)
> +{
> +	return -EINVAL;
> +}
> +

Much cleaner.

Can we have a common folio_dup_swap() function instead that does what the
variant below does? (call folio_dup_swap_pages())

> +static inline int folio_dup_swap_pages(struct folio *folio, struct page *page,
> +		unsigned long nr_pages)
>  {
>  	return -EINVAL;
>  }
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index c7e173b93e11d..28daf92839e77 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1740,9 +1740,10 @@ 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.
> - * @subpage: if not NULL, only increase the swap count of this subpage.
> + * @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
> @@ -1756,23 +1757,26 @@ 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 *subpage)
> +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);
>  
>  	VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
>  	VM_WARN_ON_FOLIO(!folio_test_swapcache(folio), folio);
>  
> -	if (subpage) {
> -		entry.val += folio_page_idx(folio, subpage);
> -		nr_pages = 1;
> -	}
> +	entry.val += folio_page_idx(folio, page);
>  
>  	return swap_dup_entries_cluster(swap_entry_to_info(entry),
>  					swp_offset(entry), nr_pages);
>  }
>  
> +int folio_dup_swap(struct folio *folio)
> +{
> +	return folio_dup_swap_pages(folio, folio_page(folio, 0),
> +				    folio_nr_pages(folio));
> +}

Can you add simplistic kerneldoc for folio_dup_swap() as well, and mostly just
link to folio_dup_swap_pages() ?

-- 
Cheers,

David