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

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

On Wed, Mar 11, 2026 at 11:12:22AM +0530, Dev Jain wrote:
>
>
> On 10/03/26 2:19 pm, Lorenzo Stoakes (Oracle) wrote:
> > 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.
>
>
> I get the essence of the point you are making.
>
> Since most callers of folio_put_swap mean it for entire folio, perhaps
> we can have folio_put_swap for these callers, and the ones which are
> not sure can call folio_put_swap_subpages? Same for folio_dup_swap.
> And since we are calling it folio_put_swap_subpages, we can retain
> the subpage parameter?

Right but I'm talking about dup variants here but the principle is the same.

>
> >
> > 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
>

Cheers, Lorenzo