4 files changed, 24 insertions, 2 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 33fe03551105..791e47ce9d27 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -251,7 +251,8 @@ struct btrfs_super_block {
 #define BTRFS_FEATURE_COMPAT_SAFE_CLEAR         0ULL
 
 #define BTRFS_FEATURE_COMPAT_RO_SUPP                    \
-        (BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE)
+        (BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE |      \
+         BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID)
 
 #define BTRFS_FEATURE_COMPAT_RO_SAFE_SET        0ULL
 #define BTRFS_FEATURE_COMPAT_RO_SAFE_CLEAR      0ULL
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) {
diff --git a/fs/btrfs/free-space-tree.c b/fs/btrfs/free-space-tree.c
index 8fd85bfbe2da..ea605ffd0e03 100644
--- a/fs/btrfs/free-space-tree.c
+++ b/fs/btrfs/free-space-tree.c
@@ -1182,6 +1182,7 @@ int btrfs_create_free_space_tree(struct btrfs_fs_info *fs_info)
         }
 
         btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE);
+        btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID);
         fs_info->creating_free_space_tree = 0;
 
         ret = btrfs_commit_transaction(trans, tree_root);
@@ -1250,6 +1251,7 @@ int btrfs_clear_free_space_tree(struct btrfs_fs_info *fs_info)
                 return PTR_ERR(trans);
 
         btrfs_clear_fs_compat_ro(fs_info, FREE_SPACE_TREE);
+        btrfs_clear_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID);
         fs_info->free_space_root = NULL;
 
         ret = clear_free_space_tree(trans, free_space_root);
diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index ac5eacd3055b..db4c253f8011 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -239,7 +239,17 @@ struct btrfs_ioctl_fs_info_args {
  * Used by:
  * struct btrfs_ioctl_feature_flags
  */
-#define BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE (1ULL << 0)
+#define BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE         (1ULL << 0)
+/*
+ * Older kernels (< 4.9) on big-endian systems produced broken free space tree
+ * bitmaps, and btrfs-progs also used to corrupt the free space tree (versions
+ * < 4.7.3).  If this bit is clear, then the free space tree cannot be trusted.
+ * btrfs-progs can also intentionally clear this bit to ask the kernel to
+ * rebuild the free space tree, however this might not work on older kernels
+ * that do not know about this bit. If not sure, clear the cache manually on
+ * first mount when booting older kernel versions.
+ */
+#define BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID   (1ULL << 1)
 
 #define BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF    (1ULL << 0)
 #define BTRFS_FEATURE_INCOMPAT_DEFAULT_SUBVOL   (1ULL << 1)