mm: swap: enable swap_entry_range_free() to drop any kind of last ref

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.

[shikemeng@huaweicloud.com: add __maybe_unused attribute for swap_is_last_ref() and update comment]
Link: https://lkml.kernel.org/r/20250410153908.612984-1-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20250325162528.68385-3-shikemeng@huaweicloud.com
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Tested-by: SeongJae Park <sj@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/swapfile.c b/mm/swapfile.c
index ea01cf0..c6b4c74 100644 (file)
--- 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);
@@ -1471,7 +1471,7 @@ 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;
@@ -1501,7 +1501,7 @@ 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;
@@ -1520,12 +1520,22 @@ fallback:
 }
 
 /*
- * Drop the last HAS_CACHE flag of swap entries, caller have to
- * ensure all entries belong to the same cgroup.
+ * Check if it's the last ref of swap entry in the freeing path.
+ * Qualified vlaue includes 1, SWAP_HAS_CACHE or SWAP_MAP_SHMEM.
  */
-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 __maybe_unused swap_is_last_ref(unsigned char count)
+{
+       return (count == SWAP_HAS_CACHE) || (count == 1) ||
+              (count == SWAP_MAP_SHMEM);
+}
+
+/*
+ * Drop the last ref of swap entries, caller have to ensure all entries
+ * belong to the same cgroup and cluster.
+ */
+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;
@@ -1538,7 +1548,7 @@ 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);
 
@@ -1561,7 +1571,7 @@ 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);
 }
@@ -1604,11 +1614,11 @@ 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);