[PATCH 3/6] mm, swap: avoid BUG_ON in relocate_cluster()

[Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>]

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);

Despite bug_on could be triggered, call free_cluster() for free cluster
only move cluster to tail of list and should be fine.

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

Fixes: 3b644773eefda ("mm, swap: reduce contention on device lock")
Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 mm/swapfile.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 425126c0a07d..fc45b9d56639 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],
-- 
2.30.0