[PATCH v3 2/8] mm: swap: enable swap_entry_range_free() to drop any kind of last ref

[Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>]

The original VM_BUG_ON only allows swap_entry_range_free() to drop last
SWAP_HAS_CACHE ref. By allowing other kind of last ref in VM_BUG_ON,
swap_entry_range_free() could be a more general-purpose function able to
handle all kind of last ref.
Following thi change, also rename swap_entry_range_free() to
swap_entries_free() and update it's comment accordingly.

This is a preparation to use swap_entries_free() to drop more kind of
last ref other than SWAP_HAS_CACHE.

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 mm/swapfile.c | 34 ++++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 5a775456e26c..76720ca76aae 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -52,9 +52,9 @@
 static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
 				 unsigned char);
 static void free_swap_count_continuations(struct swap_info_struct *);
-static void swap_entry_range_free(struct swap_info_struct *si,
-				  struct swap_cluster_info *ci,
-				  swp_entry_t entry, unsigned int nr_pages);
+static void swap_entries_free(struct swap_info_struct *si,
+			      struct swap_cluster_info *ci,
+			      swp_entry_t entry, unsigned int nr_pages);
 static void swap_range_alloc(struct swap_info_struct *si,
 			     unsigned int nr_entries);
 static bool folio_swapcache_freeable(struct folio *folio);
@@ -1463,7 +1463,7 @@ static unsigned char swap_entry_put(struct swap_info_struct *si,
 	ci = lock_cluster(si, offset);
 	usage = swap_entry_put_locked(si, offset, 1);
 	if (!usage)
-		swap_entry_range_free(si, ci, swp_entry(si->type, offset), 1);
+		swap_entries_free(si, ci, swp_entry(si->type, offset), 1);
 	unlock_cluster(ci);
 
 	return usage;
@@ -1493,7 +1493,7 @@ static bool swap_entries_put_nr(struct swap_info_struct *si,
 	for (i = 0; i < nr; i++)
 		WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
 	if (!has_cache)
-		swap_entry_range_free(si, ci, entry, nr);
+		swap_entries_free(si, ci, entry, nr);
 	unlock_cluster(ci);
 
 	return has_cache;
@@ -1512,12 +1512,18 @@ static bool swap_entries_put_nr(struct swap_info_struct *si,
 }
 
 /*
- * Drop the last HAS_CACHE flag of swap entries, caller have to
- * ensure all entries belong to the same cgroup.
+ * Drop the last ref(1, SWAP_HAS_CACHE or SWAP_MAP_SHMEM) of swap entries,
+ * caller have to ensure all entries belong to the same cgroup and cluster.
  */
-static void swap_entry_range_free(struct swap_info_struct *si,
-				  struct swap_cluster_info *ci,
-				  swp_entry_t entry, unsigned int nr_pages)
+static inline bool swap_is_last_ref(unsigned char count)
+{
+	return (count == SWAP_HAS_CACHE) || (count == 1) ||
+	       (count == SWAP_MAP_SHMEM);
+}
+
+static void swap_entries_free(struct swap_info_struct *si,
+			      struct swap_cluster_info *ci,
+			      swp_entry_t entry, unsigned int nr_pages)
 {
 	unsigned long offset = swp_offset(entry);
 	unsigned char *map = si->swap_map + offset;
@@ -1530,7 +1536,7 @@ static void swap_entry_range_free(struct swap_info_struct *si,
 
 	ci->count -= nr_pages;
 	do {
-		VM_BUG_ON(*map != SWAP_HAS_CACHE);
+		VM_BUG_ON(!swap_is_last_ref(*map));
 		*map = 0;
 	} while (++map < map_end);
 
@@ -1553,7 +1559,7 @@ static void cluster_swap_free_nr(struct swap_info_struct *si,
 	ci = lock_cluster(si, offset);
 	do {
 		if (!swap_entry_put_locked(si, offset, usage))
-			swap_entry_range_free(si, ci, swp_entry(si->type, offset), 1);
+			swap_entries_free(si, ci, swp_entry(si->type, offset), 1);
 	} while (++offset < end);
 	unlock_cluster(ci);
 }
@@ -1596,11 +1602,11 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
 
 	ci = lock_cluster(si, offset);
 	if (swap_only_has_cache(si, offset, size))
-		swap_entry_range_free(si, ci, entry, size);
+		swap_entries_free(si, ci, entry, size);
 	else {
 		for (int i = 0; i < size; i++, entry.val++) {
 			if (!swap_entry_put_locked(si, offset + i, SWAP_HAS_CACHE))
-				swap_entry_range_free(si, ci, entry, 1);
+				swap_entries_free(si, ci, entry, 1);
 		}
 	}
 	unlock_cluster(ci);
-- 
2.30.0