[RFC PATCH 2/2] swap: shmem: remove SWAP_MAP_SHMEM

[Nhat Pham <nphamcs@gmail.com>]

The SWAP_MAP_SHMEM state was introduced in the commit aaa468653b4a
("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a swap entry
belongs to shmem during swapoff.

However, swapoff has since been rewritten in the commit b56a2d8af914
("mm: rid swapoff of quadratic complexity"). Now having swap count ==
SWAP_MAP_SHMEM value is basically the same as having swap count == 1,
and swap_shmem_alloc() behaves analogously to swap_duplicate().

Remove this state and the associated helper to simplify the state
machine (both mentally and in terms of actual code). We will also have
an extra state/special value that can be repurposed (for swap entries
that never gets re-duplicated).

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 include/linux/swap.h |  6 ------
 mm/shmem.c           |  2 +-
 mm/swapfile.c        | 15 ---------------
 3 files changed, 1 insertion(+), 22 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index e6ab234be7be..017f3c03ff7a 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -232,7 +232,6 @@ enum {
 /* Special value in first swap_map */
 #define SWAP_MAP_MAX	0x3e	/* Max count */
 #define SWAP_MAP_BAD	0x3f	/* Note page is bad */
-#define SWAP_MAP_SHMEM	0xbf	/* Owned by shmem/tmpfs */
 
 /* Special value in each swap_map continuation */
 #define SWAP_CONT_MAX	0x7f	/* Max count */
@@ -482,7 +481,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry);
 extern swp_entry_t get_swap_page_of_type(int);
 extern int get_swap_pages(int n, swp_entry_t swp_entries[], int order);
 extern int add_swap_count_continuation(swp_entry_t, gfp_t);
-extern void swap_shmem_alloc(swp_entry_t, int);
 extern int swap_duplicate_nr(swp_entry_t, int);
 extern int swapcache_prepare(swp_entry_t entry, int nr);
 extern void swap_free_nr(swp_entry_t entry, int nr_pages);
@@ -549,10 +547,6 @@ static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
 	return 0;
 }
 
-static inline void swap_shmem_alloc(swp_entry_t swp, int nr)
-{
-}
-
 static inline int swap_duplicate_nr(swp_entry_t swp, int nr)
 {
 	return 0;
diff --git a/mm/shmem.c b/mm/shmem.c
index 361affdf3990..1875f2521dc6 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1559,7 +1559,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 			__GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN,
 			NULL) == 0) {
 		shmem_recalc_inode(inode, 0, nr_pages);
-		swap_shmem_alloc(swap, nr_pages);
+		swap_duplicate_nr(swap, nr_pages);
 		shmem_delete_from_page_cache(folio, swp_to_radix_entry(swap));
 
 		mutex_unlock(&shmem_swaplist_mutex);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 47a2cd5f590d..cebc244ee60f 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1381,12 +1381,6 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *si,
 	if (usage == SWAP_HAS_CACHE) {
 		VM_BUG_ON(!has_cache);
 		has_cache = 0;
-	} else if (count == SWAP_MAP_SHMEM) {
-		/*
-		 * Or we could insist on shmem.c using a special
-		 * swap_shmem_free() and free_shmem_swap_and_cache()...
-		 */
-		count = 0;
 	} else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
 		if (count == COUNT_CONTINUED) {
 			if (swap_count_continued(si, offset, count))
@@ -3686,15 +3680,6 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
 	return err;
 }
 
-/*
- * Help swapoff by noting that swap entry belongs to shmem/tmpfs
- * (in which case its reference count is never incremented).
- */
-void swap_shmem_alloc(swp_entry_t entry, int nr)
-{
-	__swap_duplicate(entry, SWAP_MAP_SHMEM, nr);
-}
-
 /**
  * swap_duplicate_nr() - Increase reference count of nr contiguous swap entries
  *                       by 1.
-- 
2.43.5