[PATCH v2 3/3] mm/swap: skip relocation of tableless clusters

[Nhat Pham <nphamcs@gmail.com>]

A stale reclaimer can reach relocate_cluster() after another allocator
re-isolates an emptied cluster and drops its locks to populate the table.
The cluster has zero count, CLUSTER_FLAG_NONE and no table, so relocation
can free it again and dereference NULL in the DEBUG_VM empty check.

    CPU0                                    CPU1
    Reclaimer                               Allocator
    alloc_swap_scan_cluster()
      cluster_reclaim_range()
        spin_unlock(&ci->lock)
        __try_to_reclaim_swap()
          swap_cache_del_folio()
            __swap_cache_del_folio()
              __swap_cache_do_del_folio()
                __swap_cluster_free_entries()
                  free_cluster()
                    __free_cluster()
                      swap_cluster_free_table()
            swap_cluster_unlock(ci)
                                            isolate_lock_cluster()
                                              swap_cluster_populate()
                                                drop ci and outer locks
                                                swap_cluster_alloc_table()
        spin_lock(&ci->lock)
      relocate_cluster()
        free_cluster()
          __free_cluster()
            swap_cluster_assert_empty()
              __swap_table_get()  /* NULL table */

Leave tableless clusters to the allocator under ci->lock. This also stops
a stale reclaimer from returning a cluster to the free list while table
population is pending, avoiding the resulting FREE-to-NONE state cycle.
The existing cluster lock serializes the table check and relocation with
population.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://lore.kernel.org/all/sashiko-outbox-203018@kernel.org/
Fixes: 07adc4cf1ecd ("mm, swap: implement dynamic allocation of swap table")
Cc: stable@vger.kernel.org
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/swapfile.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 32639f65f13c..9fd62e4ca7a0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -815,6 +815,13 @@ static void relocate_cluster(struct swap_info_struct *si,
 	if (cluster_is_discard(ci))
 		return;
 
+	/*
+	 * An allocator may still be preparing the table.
+	 * Leave the cluster to it.
+	 */
+	if (!cluster_table_is_alloced(ci))
+		return;
+
 	if (!ci->count) {
 		if (ci->flags != CLUSTER_FLAG_FREE)
 			free_cluster(si, ci);
-- 
2.53.0-Meta