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, 13 insertions, 8 deletions

diff --git a/fs/bcachefs/btree_cache.c b/fs/bcachefs/btree_cache.c
index 8b27b7e98f7d..b36bed3e1348 100644
--- a/fs/bcachefs/btree_cache.c
+++ b/fs/bcachefs/btree_cache.c
@@ -25,13 +25,15 @@ void bch2_recalc_btree_reserve(struct bch_fs *c)
 {
 	unsigned i, reserve = 16;
 
-	if (!c->btree_roots[0].b)
+	if (!c->btree_roots_known[0].b)
 		reserve += 8;
 
-	for (i = 0; i < BTREE_ID_NR; i++)
-		if (c->btree_roots[i].b)
-			reserve += min_t(unsigned, 1,
-					 c->btree_roots[i].b->c.level) * 8;
+	for (i = 0; i < btree_id_nr_alive(c); i++) {
+		struct btree_root *r = bch2_btree_id_root(c, i);
+
+		if (r->b)
+			reserve += min_t(unsigned, 1, r->b->c.level) * 8;
+	}
 
 	c->btree_cache.reserve = reserve;
 }
@@ -409,9 +411,12 @@ void bch2_fs_btree_cache_exit(struct bch_fs *c)
 
 	kvpfree(c->verify_ondisk, btree_bytes(c));
 
-	for (i = 0; i < BTREE_ID_NR; i++)
-		if (c->btree_roots[i].b)
-			list_add(&c->btree_roots[i].b->list, &bc->live);
+	for (i = 0; i < btree_id_nr_alive(c); i++) {
+		struct btree_root *r = bch2_btree_id_root(c, i);
+
+		if (r->b)
+			list_add(&r->b->list, &bc->live);
+	}
 
 	list_splice(&bc->freeable, &bc->live);