[PATCH v2 1/4] mm, swap: Fix potential NULL dereference when trying a sleep table allocation

The root cause of this issue is because multi-tables are updated in non
atomic context. To be more specific, the issue could be triggerred as
following:

swap_alloc_fast         swap_cluster_populate()
                         /* Try a sleep allocation */
                         spin_unlock(&ci->lock);
                         swap_cluster_alloc_table()
                          rcu_assign_pointer(ci->table, table);

 ci = swap_cluster_lock(si, offset)
 cluster_is_usable(ci, order）
  if (!cluster_table_is_alloced(ci)) // ok
 alloc_swap_scan_cluster()
  cluster_scan_range()
   __swap_table_get()

                          /* free table when more table allocation fails */
                          ci->memcg_table = kzalloc_obj(*ci->memcg_table,
                                                        gfp);
                          if (!ci->memcg_table)
                           swap_cluster_free_table()
                            rcu_assign_pointer(ci->table, NULL);

    table = rcu_dereference_check(ci->table, lockdep_is_held(&ci->lock));
     atomic_long_read(&table[off]); // NULL dereference

Fix the issue by making tables visible at end of swap_cluster_populate().

Fixes: 2fe7a6f5024b8 ("mm/memcg, swap: store cgroup id in cluster table directly")
Signed-off-by: Kemeng Shi <[email protected]>
---
 mm/swapfile.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 615d90867111..4561c864f806 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -466,8 +466,6 @@ static int swap_cluster_alloc_table(struct swap_cluster_info *ci, gfp_t gfp)
 	if (!table)
 		return -ENOMEM;
 
-	rcu_assign_pointer(ci->table, table);
-
 #ifdef CONFIG_MEMCG
 	if (!mem_cgroup_disabled()) {
 		VM_WARN_ON_ONCE(ci->memcg_table);
@@ -487,6 +485,12 @@ static int swap_cluster_alloc_table(struct swap_cluster_info *ci, gfp_t gfp)
 		return -ENOMEM;
 	}
 #endif
+
+	/*
+	 * Make tables visible to cluster_is_usable() after everything is
+	 * ready.
+	 */
+	rcu_assign_pointer(ci->table, table);
 	return 0;
 }
 
-- 
2.36.1