[RFC] btrfs: Allow read-only mount with corrupted extent tree

["Dāvis Mosāns" <davispuh@gmail.com>]

Currently if there's any corruption at all in extent tree
(eg. even single bit) then mounting will fail with:
"failed to read block groups: -5" (-EIO)
It happens because we immediately abort on first error when
searching in extent tree for block groups.

Now with this patch if `ignorebadroots` option is specified
then we handle such case and continue by removing already
created block groups and creating dummy block groups.

Signed-off-by: Dāvis Mosāns <davispuh@gmail.com>
---
 fs/btrfs/block-group.c | 14 ++++++++++++++
 fs/btrfs/disk-io.c     |  4 ++--
 fs/btrfs/disk-io.h     |  2 ++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 48ebc106a606..827a977614b3 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -2048,6 +2048,20 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
 	ret = check_chunk_block_group_mappings(info);
 error:
 	btrfs_free_path(path);
+
+	if (ret == -EIO && btrfs_test_opt(info, IGNOREBADROOTS)) {
+		btrfs_put_block_group_cache(info);
+		btrfs_stop_all_workers(info);
+		btrfs_free_block_groups(info);
+		ret = btrfs_init_workqueues(info, NULL);
+		if (ret)
+			return ret;
+		ret = btrfs_init_space_info(info);
+		if (ret)
+			return ret;
+		return fill_dummy_bgs(info);
+	}
+
 	return ret;
 }
 
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 07a2b4f69b10..dc744f76d075 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1955,7 +1955,7 @@ static int read_backup_root(struct btrfs_fs_info *fs_info, u8 priority)
 }
 
 /* helper to cleanup workers */
-static void btrfs_stop_all_workers(struct btrfs_fs_info *fs_info)
+void btrfs_stop_all_workers(struct btrfs_fs_info *fs_info)
 {
 	btrfs_destroy_workqueue(fs_info->fixup_workers);
 	btrfs_destroy_workqueue(fs_info->delalloc_workers);
@@ -2122,7 +2122,7 @@ static void btrfs_init_qgroup(struct btrfs_fs_info *fs_info)
 	mutex_init(&fs_info->qgroup_rescan_lock);
 }
 
-static int btrfs_init_workqueues(struct btrfs_fs_info *fs_info,
+int btrfs_init_workqueues(struct btrfs_fs_info *fs_info,
 		struct btrfs_fs_devices *fs_devices)
 {
 	u32 max_active = fs_info->thread_pool_size;
diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
index e45057c0c016..f9bfcba86a04 100644
--- a/fs/btrfs/disk-io.h
+++ b/fs/btrfs/disk-io.h
@@ -137,6 +137,8 @@ int btrfs_find_free_objectid(struct btrfs_root *root, u64 *objectid);
 int btrfs_find_highest_objectid(struct btrfs_root *root, u64 *objectid);
 int __init btrfs_end_io_wq_init(void);
 void __cold btrfs_end_io_wq_exit(void);
+void btrfs_stop_all_workers(struct btrfs_fs_info *fs_info);
+int btrfs_init_workqueues(struct btrfs_fs_info *fs_info, struct btrfs_fs_devices *fs_devices);
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 void btrfs_set_buffer_lockdep_class(u64 objectid,
-- 
2.30.2