[PATCH v2 1/1] swap: shmem: remove SWAP_MAP_SHMEM

[Nhat Pham]

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(). The only
difference of note is that swap_shmem_alloc() does not check for
-ENOMEM returned from __swap_duplicate(), but it is OK because shmem
never re-duplicates any swap entry it owns. This will stil be safe if we
use (batched) swap_duplicate() instead.

This commit adds swap_duplicate_nr(), the batched variant of
swap_duplicate(), and removes the SWAP_MAP_SHMEM state and the
associated swap_shmem_alloc() 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@xxxxxxxxx>
---
 include/linux/swap.h | 16 ++++++++--------
 mm/shmem.c           |  2 +-
 mm/swapfile.c        | 41 +++++++++++++++++++++--------------------
 3 files changed, 30 insertions(+), 29 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index ca533b478c21..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 */
@@ -482,8 +481,7 @@ 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(swp_entry_t);
+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);
 extern void swapcache_free_entries(swp_entry_t *entries, int n);
@@ -549,11 +547,7 @@ 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(swp_entry_t swp)
+static inline int swap_duplicate_nr(swp_entry_t swp, int nr)
 {
 	return 0;
 }
@@ -606,6 +600,12 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
 }
 #endif /* CONFIG_SWAP */
 
+static inline int swap_duplicate(swp_entry_t entry)
+{
+	return swap_duplicate_nr(entry, 1);
+}
+
+
 static inline void free_swap_and_cache(swp_entry_t entry)
 {
 	free_swap_and_cache_nr(entry, 1);
diff --git a/mm/shmem.c b/mm/shmem.c
index 0613421e09e7..e3f72f99be32 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1561,7 +1561,7 @@ 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 0cded32414a1..9bb94e618914 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))
@@ -3626,7 +3620,6 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
 
 	offset = swp_offset(entry);
 	VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
-	VM_WARN_ON(usage == 1 && nr > 1);
 	ci = lock_cluster_or_swap_info(si, offset);
 
 	err = 0;
@@ -3652,6 +3645,13 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
 				err = -EEXIST;
 		} else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX) {
 			err = -EINVAL;
+		} else {
+			/*
+			 * The only swap_duplicate_nr() caller that passes nr > 1 is shmem,
+			 * who never re-duplicates any swap entry it owns. So this should
+			 * not happen.
+			 */
+			VM_WARN_ON(nr > 1 && (count & ~COUNT_CONTINUED) == SWAP_MAP_MAX);
 		}
 
 		if (err)
@@ -3686,27 +3686,28 @@ 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);
-}
-
-/*
- * Increase reference count of swap entry by 1.
+/**
+ * swap_duplicate_nr() - Increase reference count of nr contiguous swap entries
+ *                       by 1.
+ *
+ * @entry: first swap entry from which we want to increase the refcount.
+ * @nr: Number of entries in range.
+ *
  * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required
  * but could not be atomically allocated.  Returns 0, just as if it succeeded,
  * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which
  * might occur if a page table entry has got corrupted.
+ *
+ * Note that we are currently not handling the case where nr > 1 and we need to
+ * add swap count continuation. This is OK, because no such user exists - shmem
+ * is the only user that can pass nr > 1, and it never re-duplicates any swap
+ * entry it owns.
  */
-int swap_duplicate(swp_entry_t entry)
+int swap_duplicate_nr(swp_entry_t entry, int nr)
 {
 	int err = 0;
 
-	while (!err && __swap_duplicate(entry, 1, 1) == -ENOMEM)
+	while (!err && __swap_duplicate(entry, 1, nr) == -ENOMEM)
 		err = add_swap_count_continuation(entry, GFP_ATOMIC);
 	return err;
 }
-- 
2.43.5