[PATCH v7 04/12] btrfs: create free space tree on ro->rw remount

[Boris Burkov <boris@bur.io>]

When a user attempts to remount a btrfs filesystem with
'mount -o remount,space_cache=v2', that operation silently succeeds.
Unfortunately, this is misleading, because the remount does not create
the free space tree. /proc/mounts will incorrectly show space_cache=v2,
but on the next mount, the file system will revert to the old
space_cache.

For now, we handle only the easier case, where the existing mount is
read-only and the new mount is read-write. In that case, we can create
the free space tree without contending with the block groups changing
as we go.

Signed-off-by: Boris Burkov <boris@bur.io>
---
 fs/btrfs/disk-io.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 2cf81a4e9393..d934eb80ff49 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2912,6 +2912,17 @@ int btrfs_mount_rw(struct btrfs_fs_info *fs_info)
 		goto out;
 	}
 
+	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE) &&
+	    !btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
+		btrfs_info(fs_info, "creating free space tree");
+		ret = btrfs_create_free_space_tree(fs_info);
+		if (ret) {
+			btrfs_warn(fs_info,
+				"failed to create free space tree: %d", ret);
+			goto out;
+		}
+	}
+
 	ret = btrfs_resume_balance_async(fs_info);
 	if (ret)
 		goto out;
@@ -3376,18 +3387,6 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		}
 	}
 
-	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE) &&
-	    !btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
-		btrfs_info(fs_info, "creating free space tree");
-		ret = btrfs_create_free_space_tree(fs_info);
-		if (ret) {
-			btrfs_warn(fs_info,
-				"failed to create free space tree: %d", ret);
-			close_ctree(fs_info);
-			return ret;
-		}
-	}
-
 	ret = btrfs_mount_rw(fs_info);
 	if (ret) {
 		close_ctree(fs_info);
-- 
2.24.1