[PATCH] mm/zswap: fix double decrement of the LRU node count on writeback

[Kaitao Cheng <kaitao.cheng@linux.dev>]

From: Kaitao Cheng <chengkaitao@kylinos.cn>

Successful writeback calls zswap_entry_free(), which removes the entry
from the LRU and decrements both the sublist and node item counts.
shrink_memcg_cb() then returns LRU_REMOVED_RETRY, causing the LRU walker
to decrement the node count again.

The affected call path before this change is:

  __list_lru_walk_one()
    shrink_memcg_cb()
      zswap_writeback_entry()
        zswap_entry_free()
          zswap_lru_del()
            list_lru_del()
              __list_lru_del()
                atomic_long_dec()  /* first node count decrement */
      return LRU_REMOVED_RETRY
    atomic_long_dec()              /* second node count decrement */

Add zswap_lru_del_nocnt() to remove the entry and update only the sublist
count, leaving node accounting to the walker. Split data cleanup into
__zswap_entry_free() so writeback can free the entry without performing
another LRU deletion. Other free paths retain their existing accounting.

Fixes: 5878303c5353 ("mm/zswap: fix race between lru writeback and swapoff")
Link: https://sashiko.dev/#/bug/linux-68d2ca49-23be-4504-9a75-b8526ec92388
Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
---
 mm/zswap.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index ad01c691a263..d0c98daddf2e 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -719,6 +719,22 @@ static void zswap_lru_del(struct zswap_entry *entry)
 	rcu_read_unlock();
 }
 
+/* Like zswap_lru_del(), but does not decrement the node item count. */
+static void zswap_lru_del_nocnt(struct zswap_entry *entry)
+{
+	int nid = entry_to_nid(entry);
+	struct mem_cgroup *memcg;
+	struct list_lru_one *l;
+
+	rcu_read_lock();
+	memcg = obj_cgroup_memcg(entry->objcg);
+	l = list_lru_lock(&zswap_list_lru, nid, &memcg);
+	if (!list_empty(&entry->lru))
+		list_lru_isolate(l, &entry->lru);
+	list_lru_unlock(l);
+	rcu_read_unlock();
+}
+
 void zswap_lruvec_state_init(struct lruvec *lruvec)
 {
 	atomic_long_set(&lruvec->zswap_lruvec_state.nr_disk_swapins, 0);
@@ -781,11 +797,10 @@ static void zswap_entry_cache_free(struct zswap_entry *entry)
  * Carries out the common pattern of freeing an entry's zsmalloc allocation,
  * freeing the entry itself, and decrementing the number of stored pages.
  */
-static void zswap_entry_free(struct zswap_entry *entry)
+static void __zswap_entry_free(struct zswap_entry *entry)
 {
 	struct zswap_pool *pool = zswap_entry_pool(entry);
 
-	zswap_lru_del(entry);
 	if (!WARN_ON_ONCE(!pool)) {
 		zs_free(pool->zs_pool, entry->handle);
 		zswap_pool_put(pool);
@@ -800,6 +815,12 @@ static void zswap_entry_free(struct zswap_entry *entry)
 	atomic_long_dec(&zswap_stored_pages);
 }
 
+static void zswap_entry_free(struct zswap_entry *entry)
+{
+	zswap_lru_del(entry);
+	__zswap_entry_free(entry);
+}
+
 /*********************************
 * compressed storage functions
 **********************************/
@@ -1119,7 +1140,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	if (entry->objcg)
 		count_objcg_events(entry->objcg, ZSWPWB, 1);
 
-	zswap_entry_free(entry);
+	zswap_lru_del_nocnt(entry);
+	__zswap_entry_free(entry);
 
 	/* folio is up to date */
 	folio_mark_uptodate(folio);
-- 
2.53.0