Re: [PATCH 6/9] mm/swapfile: Make folio_dup_swap batchable

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Tue, Mar 10, 2026 at 01:00:10PM +0530, Dev Jain wrote:
> Teach folio_dup_swap 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, and explicitly pass subpage != NULL, and the number of
> pages required.

You've made the interface more confusing? Now we can update multiple subpages
but specify only one? :)

Let's try to actually refactor this into something sane... see below.

>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/rmap.c     |  2 +-
>  mm/shmem.c    |  2 +-
>  mm/swap.h     |  5 +++--
>  mm/swapfile.c | 12 +++++-------
>  4 files changed, 10 insertions(+), 11 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index dd638429c963e..f6d5b187cf09b 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2282,7 +2282,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  				goto discard;
>  			}
>
> -			if (folio_dup_swap(folio, subpage) < 0) {
> +			if (folio_dup_swap(folio, subpage, 1) < 0) {
>  				set_pte_at(mm, address, pvmw.pte, pteval);
>  				goto walk_abort;
>  			}
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 5e7dcf5bc5d3c..86ee34c9b40b3 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1695,7 +1695,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>  			spin_unlock(&shmem_swaplist_lock);
>  		}
>
> -		folio_dup_swap(folio, NULL);
> +		folio_dup_swap(folio, folio_page(folio, 0), folio_nr_pages(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..d9cb58ebbddd1 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -206,7 +206,7 @@ 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, struct page *subpage, unsigned int nr_pages);
>  void folio_put_swap(struct folio *folio, struct page *subpage);
>
>  /* For internal use */
> @@ -390,7 +390,8 @@ 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(struct folio *folio, struct page *page,
> +				 unsigned int nr_pages)
>  {
>  	return -EINVAL;
>  }
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 915bc93964dbd..eaf61ae6c3817 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1738,7 +1738,8 @@ int folio_alloc_swap(struct folio *folio)
>  /**
>   * folio_dup_swap() - 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.
> + * @subpage: Increase the swap count of this subpage till nr number of
> + * pages forward.

(Obviously also Kairui's point about missing entry in kdoc)

This is REALLY confusing sorry. And this interface is just a horror show.

Before we had subpage == only increase the swap count of the subpage.

Now subpage = the first subpage at which we do that? Please, no.

You just need to rework this interface in general, this is a hack.

Something like:

int __folio_dup_swap(struct folio *folio, unsigned int subpage_start_index,
		     unsigned int nr_subpages)
{
	...
}

...

int folio_dup_swap_subpage(struct folio *folio, struct page *subpage)
{
	return __folio_dup_swap(folio, folio_page_idx(folio, subpage), 1);
}

int folio_dup_swap(struct folio *folio)
{
	return __folio_dup_swap(folio, 0, folio_nr_pages(folio));
}

Or something like that.

We're definitely _not_ keeping the subpage parameter like that and hacking on
batching, PLEASE.

>   *
>   * 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
> @@ -1752,18 +1753,15 @@ 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 *subpage)
> +int folio_dup_swap(struct folio *folio, struct page *subpage,
> +		   unsigned int 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, subpage);
>
>  	return swap_dup_entries_cluster(swap_entry_to_info(entry),
>  					swp_offset(entry), nr_pages);
> --
> 2.34.1
>

Thanks, Lorenzo