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, 21 insertions, 1 deletions

diff --git a/fs/bcachefs/btree_cache.h b/fs/bcachefs/btree_cache.h
index ea375ae25a70..4c11975208b3 100644
--- a/fs/bcachefs/btree_cache.h
+++ b/fs/bcachefs/btree_cache.h
@@ -97,7 +97,27 @@ static inline unsigned btree_blocks(struct bch_fs *c)
 	(BTREE_FOREGROUND_MERGE_THRESHOLD(c) +			\
 	 (BTREE_FOREGROUND_MERGE_THRESHOLD(c) >> 2))
 
-#define btree_node_root(_c, _b)	((_c)->btree_roots[(_b)->c.btree_id].b)
+static inline unsigned btree_id_nr_alive(struct bch_fs *c)
+{
+	return BTREE_ID_NR + c->btree_roots_extra.nr;
+}
+
+static inline struct btree_root *bch2_btree_id_root(struct bch_fs *c, unsigned id)
+{
+	if (likely(id < BTREE_ID_NR)) {
+		return &c->btree_roots_known[id];
+	} else {
+		unsigned idx = id - BTREE_ID_NR;
+
+		EBUG_ON(idx >= c->btree_roots_extra.nr);
+		return &c->btree_roots_extra.data[idx];
+	}
+}
+
+static inline struct btree *btree_node_root(struct bch_fs *c, struct btree *b)
+{
+	return bch2_btree_id_root(c, b->c.btree_id)->b;
+}
 
 void bch2_btree_node_to_text(struct printbuf *, struct bch_fs *,
 			     const struct btree *);