Re: [PATCH 7/9] mm/swapfile: Make folio_put_swap batchable

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

On Tue, Mar 10, 2026 at 01:00:11PM +0530, Dev Jain wrote:
> Teach folio_put_swap to handle a batch of consecutive pages. Note that
> folio_put_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.
>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/memory.c   |  6 +++---
>  mm/rmap.c     |  4 ++--
>  mm/shmem.c    |  6 +++---
>  mm/swap.h     |  5 +++--
>  mm/swapfile.c | 13 +++++--------
>  5 files changed, 16 insertions(+), 18 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 768646c0b3b6a..8249a9b7083ab 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5002,7 +5002,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	if (unlikely(folio != swapcache)) {
>  		folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
>  		folio_add_lru_vma(folio, vma);
> -		folio_put_swap(swapcache, NULL);
> +		folio_put_swap(swapcache, folio_page(swapcache, 0), folio_nr_pages(swapcache));

Lord above HELPER. HELPER :) please.

I think in general, let's have the same refactoring in folio_put_swap() as I
suggested for folio_dup_swap().

I'm not sure where this hellish subpage interface came from, I mean there must
be a good reason but it seems kinda horrible.

>  	} else if (!folio_test_anon(folio)) {
>  		/*
>  		 * We currently only expect !anon folios that are fully
> @@ -5011,12 +5011,12 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		VM_WARN_ON_ONCE_FOLIO(folio_nr_pages(folio) != nr_pages, folio);
>  		VM_WARN_ON_ONCE_FOLIO(folio_mapped(folio), folio);
>  		folio_add_new_anon_rmap(folio, vma, address, rmap_flags);
> -		folio_put_swap(folio, NULL);
> +		folio_put_swap(folio, folio_page(folio, 0), folio_nr_pages(folio));
>  	} else {
>  		VM_WARN_ON_ONCE(nr_pages != 1 && nr_pages != folio_nr_pages(folio));
>  		folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, address,
>  					 rmap_flags);
> -		folio_put_swap(folio, nr_pages == 1 ? page : NULL);
> +		folio_put_swap(folio, page, nr_pages);

I'm confused as to why some callers use folio_nr_pages() and others nr_pages...

>  	}
>
>  	VM_BUG_ON(!folio_test_anon(folio) ||
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f6d5b187cf09b..42f6b00cced01 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2293,7 +2293,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  			 * so we'll not check/care.
>  			 */
>  			if (arch_unmap_one(mm, vma, address, pteval) < 0) {
> -				folio_put_swap(folio, subpage);
> +				folio_put_swap(folio, subpage, 1);

Again, as with the folio_dup_swap() refactoring I suggested in previous patch,
something like folio_dup_swap_subpage() would be good here right?

Like folio_put_swap_subpage(folio, subpage)...


>  				set_pte_at(mm, address, pvmw.pte, pteval);
>  				goto walk_abort;
>  			}
> @@ -2301,7 +2301,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  			/* See folio_try_share_anon_rmap(): clear PTE first. */
>  			if (anon_exclusive &&
>  			    folio_try_share_anon_rmap_pte(folio, subpage)) {
> -				folio_put_swap(folio, subpage);
> +				folio_put_swap(folio, subpage, 1);
>  				set_pte_at(mm, address, pvmw.pte, pteval);
>  				goto walk_abort;
>  			}
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 86ee34c9b40b3..d9d216ea28ecb 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1716,7 +1716,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>  		/* Swap entry might be erased by racing shmem_free_swap() */
>  		if (!error) {
>  			shmem_recalc_inode(inode, 0, -nr_pages);
> -			folio_put_swap(folio, NULL);
> +			folio_put_swap(folio, folio_page(folio, 0), folio_nr_pages(folio));
>  		}
>
>  		/*
> @@ -2196,7 +2196,7 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
>
>  	nr_pages = folio_nr_pages(folio);
>  	folio_wait_writeback(folio);
> -	folio_put_swap(folio, NULL);
> +	folio_put_swap(folio, folio_page(folio, 0), folio_nr_pages(folio));
>  	swap_cache_del_folio(folio);
>  	/*
>  	 * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
> @@ -2426,7 +2426,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>  	if (sgp == SGP_WRITE)
>  		folio_mark_accessed(folio);
>
> -	folio_put_swap(folio, NULL);
> +	folio_put_swap(folio, folio_page(folio, 0), folio_nr_pages(folio));

Again, for emphasis, please do not repeatedly open code the exact same thing all
over the place, 'don't repeat yourself' is a really good principle in
programming in general. Now if one of these callers gets updated and the others
not we're in a mess.

Abstract this please :)

>  	swap_cache_del_folio(folio);
>  	folio_mark_dirty(folio);
>  	put_swap_device(si);
> diff --git a/mm/swap.h b/mm/swap.h
> index d9cb58ebbddd1..73fd9faa67608 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -207,7 +207,7 @@ extern int swap_retry_table_alloc(swp_entry_t entry, gfp_t gfp);
>   */
>  int folio_alloc_swap(struct folio *folio);
>  int folio_dup_swap(struct folio *folio, struct page *subpage, unsigned int nr_pages);
> -void folio_put_swap(struct folio *folio, struct page *subpage);
> +void folio_put_swap(struct folio *folio, struct page *subpage, unsigned int nr_pages);
>
>  /* For internal use */
>  extern void __swap_cluster_free_entries(struct swap_info_struct *si,
> @@ -396,7 +396,8 @@ static inline int folio_dup_swap(struct folio *folio, struct page *page,
>  	return -EINVAL;
>  }
>
> -static inline void folio_put_swap(struct folio *folio, struct page *page)
> +static inline void folio_put_swap(struct folio *folio, struct page *page,
> +				  unsigned int nr_pages)
>  {
>  }
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index eaf61ae6c3817..c66aa6d15d479 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1770,25 +1770,22 @@ int folio_dup_swap(struct folio *folio, struct page *subpage,
>  /**
>   * folio_put_swap() - Decrease swap count of swap entries of a folio.
>   * @folio: folio with swap entries bounded, must be in swap cache and locked.
> - * @subpage: if not NULL, only decrease the swap count of this subpage.
> + * @subpage: decrease the swap count of this subpage till nr_pages.

Again no nr_pages entry.

>   *
>   * This won't free the swap slots even if swap count drops to zero, they are
>   * still pinned by the swap cache. User may call folio_free_swap to free them.
>   * Context: Caller must ensure the folio is locked and in the swap cache.
>   */
> -void folio_put_swap(struct folio *folio, struct page *subpage)
> +void folio_put_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);
>  	struct swap_info_struct *si = __swap_entry_to_info(entry);
>
>  	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);
>
>  	swap_put_entries_cluster(si, swp_offset(entry), nr_pages, false);

And yeah exact same comments re: refactoring as per folio_dup_swap().

>  }
> @@ -2334,7 +2331,7 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>  		new_pte = pte_mkuffd_wp(new_pte);
>  setpte:
>  	set_pte_at(vma->vm_mm, addr, pte, new_pte);
> -	folio_put_swap(swapcache, folio_file_page(swapcache, swp_offset(entry)));
> +	folio_put_swap(swapcache, folio_file_page(swapcache, swp_offset(entry)), 1);
>  out:
>  	if (pte)
>  		pte_unmap_unlock(pte, ptl);
> --
> 2.34.1
>

Thanks, Lorenzo