+ mm-swap-skip-relocation-of-tableless-clusters.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/swap: skip relocation of tableless clusters
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-swap-skip-relocation-of-tableless-clusters.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-swap-skip-relocation-of-tableless-clusters.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Nhat Pham <nphamcs@gmail.com>
Subject: mm/swap: skip relocation of tableless clusters
Date: Tue, 10 Nov 2026 07:14:07 -0800

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.

Link: https://lore.kernel.org/20261110151407.789960-4-nphamcs@gmail.com
Fixes: 07adc4cf1ecd ("mm, swap: implement dynamic allocation of swap table")
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/sashiko-outbox-203018@kernel.org/
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christopher Li <chrisl@kernel.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Youngjun Park <youngjun.park@lge.com>
Cc: <ryncsn@gmail.com>
Cc: <stable@vger.kernel.org>
---

 mm/swapfile.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/mm/swapfile.c~mm-swap-skip-relocation-of-tableless-clusters
+++ a/mm/swapfile.c
@@ -815,6 +815,13 @@ static void relocate_cluster(struct swap
 	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);
_

Patches currently in -mm which might be from nphamcs@gmail.com are

mm-vmscan-allow-writeout-of-swapcache-folios-without-anon-mappings.patch
mm-swap-publish-cluster-tables-after-reacquiring-allocator-locks.patch
mm-swap-skip-relocation-of-tableless-clusters.patch