bcachefs: Replace bucket_valid() asserts in bucket lookup with proper checks

The bucket_gens array and gc_buckets array known their own size; we
should be using those members, and returning an error.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_gc.c

@@ -990,6 +990,8 @@ static int bch2_gc_alloc_start(struct bch_fs *c)
 
 		buckets->first_bucket	= ca->mi.first_bucket;
 		buckets->nbuckets	= ca->mi.nbuckets;
+		buckets->nbuckets_minus_first =
+			buckets->nbuckets - buckets->first_bucket;
 		rcu_assign_pointer(ca->buckets_gc, buckets);
 	}
 

fs/bcachefs/buckets.c

@@ -1612,6 +1612,8 @@ int bch2_dev_buckets_resize(struct bch_fs *c, struct bch_dev *ca, u64 nbuckets)
 
 	bucket_gens->first_bucket = ca->mi.first_bucket;
 	bucket_gens->nbuckets	= nbuckets;
+	bucket_gens->nbuckets_minus_first =
+		bucket_gens->nbuckets - bucket_gens->first_bucket;
 
 	if (resize) {
 		down_write(&c->gc_lock);

fs/bcachefs/buckets.h

@@ -93,7 +93,8 @@ static inline struct bucket *gc_bucket(struct bch_dev *ca, size_t b)
 {
 	struct bucket_array *buckets = gc_bucket_array(ca);
 
-	BUG_ON(!bucket_valid(ca, b));
+	if (b - buckets->first_bucket >= buckets->nbuckets_minus_first)
+		return NULL;
 	return buckets->b + b;
 }
 
@@ -110,7 +111,8 @@ static inline u8 *bucket_gen(struct bch_dev *ca, size_t b)
 {
 	struct bucket_gens *gens = bucket_gens(ca);
 
-	BUG_ON(!bucket_valid(ca, b));
+	if (b - gens->first_bucket >= gens->nbuckets_minus_first)
+		return NULL;
 	return gens->b + b;
 }
 

fs/bcachefs/buckets_types.h

@@ -22,6 +22,7 @@ struct bucket_array {
 	struct rcu_head		rcu;
 	u16			first_bucket;
 	size_t			nbuckets;
+	size_t			nbuckets_minus_first;
 	struct bucket		b[];
 };
 
@@ -29,6 +30,7 @@ struct bucket_gens {
 	struct rcu_head		rcu;
 	u16			first_bucket;
 	size_t			nbuckets;
+	size_t			nbuckets_minus_first;
 	u8			b[];
 };