[PATCH v2 08/19] mm/shmem, swap: remove SWAP_MAP_SHMEM

[Kairui Song <ryncsn@gmail.com>]

From: 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(). 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@gmail.com>
Signed-off-by: Kairui Song <kasong@tencent.com>
---
 include/linux/swap.h | 15 +++++++--------
 mm/shmem.c           |  2 +-
 mm/swapfile.c        | 42 +++++++++++++++++-------------------------
 3 files changed, 25 insertions(+), 34 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 38ca3df68716..bf72b548a96d 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -230,7 +230,6 @@ 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 */
@@ -458,8 +457,7 @@ bool folio_free_swap(struct folio *folio);
 void put_swap_folio(struct folio *folio, swp_entry_t entry);
 extern swp_entry_t get_swap_page_of_type(int);
 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 entry, int nr);
 extern int swapcache_prepare(swp_entry_t entry, int nr);
 extern void swap_free_nr(swp_entry_t entry, int nr_pages);
 extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
@@ -514,11 +512,7 @@ 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_pages)
 {
 	return 0;
 }
@@ -569,6 +563,11 @@ 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 8b711b8bcdba..0b0af71ce5e8 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1654,7 +1654,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
 			spin_unlock(&shmem_swaplist_lock);
 		}
 
-		swap_shmem_alloc(folio->swap, nr_pages);
+		swap_duplicate_nr(folio->swap, nr_pages);
 		shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap));
 
 		BUG_ON(folio_mapped(folio));
diff --git a/mm/swapfile.c b/mm/swapfile.c
index a433445e9225..580adf9d93bd 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -201,7 +201,7 @@ static bool swap_is_last_map(struct swap_info_struct *si,
 	unsigned char *map_end = map + nr_pages;
 	unsigned char count = *map;
 
-	if (swap_count(count) != 1 && swap_count(count) != SWAP_MAP_SHMEM)
+	if (swap_count(count) != 1)
 		return false;
 
 	while (++map < map_end) {
@@ -1522,12 +1522,6 @@ static unsigned char swap_entry_put_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))
@@ -1625,7 +1619,7 @@ static bool swap_entries_put_map(struct swap_info_struct *si,
 	if (nr <= 1)
 		goto fallback;
 	count = swap_count(data_race(si->swap_map[offset]));
-	if (count != 1 && count != SWAP_MAP_SHMEM)
+	if (count != 1)
 		goto fallback;
 
 	ci = swap_cluster_lock(si, offset);
@@ -1679,12 +1673,10 @@ static bool swap_entries_put_map_nr(struct swap_info_struct *si,
 
 /*
  * Check if it's the last ref of swap entry in the freeing path.
- * Qualified value includes 1, SWAP_HAS_CACHE or SWAP_MAP_SHMEM.
  */
 static inline bool __maybe_unused swap_is_last_ref(unsigned char count)
 {
-	return (count == SWAP_HAS_CACHE) || (count == 1) ||
-	       (count == SWAP_MAP_SHMEM);
+	return (count == SWAP_HAS_CACHE) || (count == 1);
 }
 
 /*
@@ -3678,7 +3670,6 @@ 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 = swap_cluster_lock(si, offset);
 
 	err = 0;
@@ -3738,27 +3729,28 @@ 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;
 }

-- 
2.51.2