Btrfs: fix an oops when we fail to relocate tree blocks

During balance test, we hit an oops:
[ 2013.841551] kernel BUG at fs/btrfs/relocation.c:1174!

The problem is that if we fail to relocate tree blocks, we should
update backref cache, otherwise, some pending nodes are not updated
while snapshot check @cache->last_trans is within one transaction
and won't update it and then oops happen.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 9189f9e3c359..07b3b36f40ee 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -4009,6 +4009,12 @@ restart:
                 if (!RB_EMPTY_ROOT(&blocks)) {
                         ret = relocate_tree_blocks(trans, rc, &blocks);
                         if (ret < 0) {
+                                /*
+                                 * if we fail to relocate tree blocks, force to update
+                                 * backref cache when committing transaction.
+                                 */
+                                rc->backref_cache.last_trans = trans->transid - 1;
+
                                 if (ret != -EAGAIN) {
                                         err = ret;
                                         break;