bcachefs: Allow for unknown btree IDs

We need to allow filesystems with metadata from newer versions to be
mountable and usable by older versions.

This patch enables us to roll out new btrees without a new major version
number; we can now handle btree roots for unknown btree types.

The unknown btree roots will be retained, and fsck (including
backpointers) will check them, the same as other btree types.

We add a dynamic array for the extra, unknown btree roots, in addition
to the fixed size btree root array, and add new helpers for looking up
btree roots.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/bcachefs/bcachefs.h b/fs/bcachefs/bcachefs.h
index b8d50fe64b3c..a8488d4e18e8 100644
--- a/fs/bcachefs/bcachefs.h
+++ b/fs/bcachefs/bcachefs.h
@@ -749,7 +749,8 @@ struct bch_fs {
 	struct bio_set		btree_bio;
 	struct workqueue_struct	*io_complete_wq;
 
-	struct btree_root	btree_roots[BTREE_ID_NR];
+	struct btree_root	btree_roots_known[BTREE_ID_NR];
+	DARRAY(struct btree_root) btree_roots_extra;
 	struct mutex		btree_root_lock;
 
 	struct btree_cache	btree_cache;