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mm, swap: avoid BUG_ON in relocate_cluster()

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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: 3b644773ee ("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>
This commit is contained in:
Kemeng Shi
2025-02-23 00:08:47 +08:00
committed by Andrew Morton
parent 7a2e7ae5d1
commit 57d910cffa
1 changed files with 2 additions and 1 deletions
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3
mm/swapfile.c
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@@ -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],