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) {
 				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;
+}
+
+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));
+}
+
 /**
  * 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.
-- 
2.34.1