[PATCH v2] btrfs: replace BUG_ON() with error return in cache_save_setup()

[Teng Liu <27rabbitlt@gmail.com>]

In cache_save_setup(), if create_free_space_inode() succeeds but the
subsequent lookup_free_space_inode() still fails on retry, the
BUG_ON(retries) will crash the kernel. This can happen due to I/O
errors or transient failures, not just programming bugs.

Replace the BUG_ON with proper error handling that returns the original
error code through the existing cleanup path. The callers already handle
this gracefully: disk_cache_state defaults to BTRFS_DC_ERROR, so the
space cache simply won't be written for that block group.

Signed-off-by: Teng Liu <27rabbitlt@gmail.com>
---
Changes in v2:
  - Use btrfs_err() instead of btrfs_debug() (Qu Wenruo)
  - Use PTR_ERR(inode) instead of hardcoded -EIO (Qu Wenruo)
  - Include error code in the log message (Qu Wenruo)

 fs/btrfs/block-group.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index ebf507909..89bfc9db0 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -3343,7 +3343,13 @@ static int cache_save_setup(struct btrfs_block_group *block_group,
 	}
 
 	if (IS_ERR(inode)) {
-		BUG_ON(retries);
+		if (retries) {
+			ret = PTR_ERR(inode);
+			btrfs_err(fs_info,
+				  "failed to lookup free space inode after creation for block group %llu: %d",
+				  block_group->start, ret);
+			goto out_free;
+		}
 		retries++;
 
 		if (block_group->ro)
-- 
2.53.0