Btrfs: catch invalid free space trees

There are two separate issues that can lead to corrupted free space
trees.

1. The free space tree bitmaps had an endianness issue on big-endian
   systems which is fixed by an earlier patch in this series.
2. btrfs-progs before v4.7.3 modified filesystems without updating the
   free space tree.

To catch both of these issues at once, we need to force the free space
tree to be rebuilt. To do so, add a FREE_SPACE_TREE_VALID compat_ro bit.
If the bit isn't set, we know that it was either produced by a broken
big-endian kernel or may have been corrupted by btrfs-progs.

This also provides us with a way to add rudimentary read-write support
for the free space tree to btrfs-progs: it can just clear this bit and
have the kernel rebuild the free space tree.

Cc: stable@vger.kernel.org # 4.5+
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index c0bfc6ce5f06..3dede6d53bad 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2566,6 +2566,7 @@ int open_ctree(struct super_block *sb,
         int num_backups_tried = 0;
         int backup_index = 0;
         int max_active;
+        int clear_free_space_tree = 0;
 
         tree_root = fs_info->tree_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
         chunk_root = fs_info->chunk_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
@@ -3131,6 +3132,14 @@ retry_root_backup:
 
         if (btrfs_test_opt(fs_info, CLEAR_CACHE) &&
             btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
+                clear_free_space_tree = 1;
+        } else if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
+                   !btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID)) {
+                btrfs_warn(fs_info, "free space tree is invalid");
+                clear_free_space_tree = 1;
+        }
+
+        if (clear_free_space_tree) {
                 btrfs_info(fs_info, "clearing free space tree");
                 ret = btrfs_clear_free_space_tree(fs_info);
                 if (ret) {