From: Kemeng Shi <shikemeng@huaweicloud.com>
Date: Sat, 22 Feb 2025 16:08:47 +0000 (+0800)
Subject: mm, swap: avoid BUG_ON in relocate_cluster()
X-Git-Tag: v6.14-rc6~6^2~18
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=57d910cffaa0be981f558ff603e2d896b36b8241;p=thirdparty%2Fkernel%2Fstable.git

mm, swap: avoid BUG_ON in relocate_cluster()

If allocation is racy with swapoff, we may call free_cluster for cluster
already in free list and trigger BUG_ON() as following:

Allocation                        Swapoff
cluster_alloc_swap_entry
 ...
 /* may get a free cluster with offset */
 offset = xxx;
 if (offset)
  ci = lock_cluster(si, offset);

                                  ...
                                   del_from_avail_list(p, true);
                                    si->flags &= ~SWP_WRITEOK;

  alloc_swap_scan_cluster(si, ci, ...)
   ...
   /* failed to alloc entry from free entry */
   if (!cluster_alloc_range(...))
    break;
   ...
   /* add back a free cluster */
   relocate_cluster(si, ci);
    if (!ci->count)
     free_cluster(si, ci);
      VM_BUG_ON(ci->flags == CLUSTER_FLAG_FREE);

To prevent the BUG_ON(), call free_cluster() for free cluster to move the
cluster to tail of list.

Check cluster is not free before calling free_cluster() in
relocate_cluster() to avoid BUG_ON().

Link: https://lkml.kernel.org/r/20250222160850.505274-4-shikemeng@huaweicloud.com
Fixes: 3b644773eefd ("mm, swap: reduce contention on device lock")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: 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 6460b6cb36c99..df7c4e8b089ca 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -653,7 +653,8 @@ static void relocate_cluster(struct swap_info_struct *si,
 		return;
 
 	if (!ci->count) {
-		free_cluster(si, ci);
+		if (ci->flags != CLUSTER_FLAG_FREE)
+			free_cluster(si, ci);
 	} else if (ci->count != SWAPFILE_CLUSTER) {
 		if (ci->flags != CLUSTER_FLAG_FRAG)
 			move_cluster(si, ci, &si->frag_clusters[ci->order],