bcachefs: Walk leaf to root in btree_gc

Next change will move gc_alloc_start initialization into the alloc
trigger, so we have to mark those first.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_gc.c

@@ -635,13 +635,27 @@ static int bch2_gc_mark_key(struct btree_trans *trans, enum btree_id btree_id,
 static int bch2_gc_btree(struct btree_trans *trans, enum btree_id btree, bool initial)
 {
 	struct bch_fs *c = trans->c;
-	int level = 0, target_depth = btree_node_type_needs_gc(__btree_node_type(0, btree)) ? 0 : 1;
+	unsigned target_depth = btree_node_type_needs_gc(__btree_node_type(0, btree)) ? 0 : 1;
 	int ret = 0;
 
 	/* We need to make sure every leaf node is readable before going RW */
 	if (initial)
 		target_depth = 0;
 
+	for (unsigned level = target_depth; level < BTREE_MAX_DEPTH; level++) {
+		struct btree *prev = NULL;
+		struct btree_iter iter;
+		bch2_trans_node_iter_init(trans, &iter, btree, POS_MIN, 0, level,
+					  BTREE_ITER_prefetch);
+
+		ret = for_each_btree_key_continue(trans, iter, 0, k, ({
+			gc_pos_set(c, gc_pos_btree(btree, level, k.k->p));
+			bch2_gc_mark_key(trans, btree, level, &prev, &iter, k, initial);
+		}));
+		if (ret)
+			goto err;
+	}
+
 	/* root */
 	do {
 retry_root:
@@ -663,28 +677,11 @@ static int bch2_gc_btree(struct btree_trans *trans, enum btree_id btree, bool in
 		gc_pos_set(c, gc_pos_btree(btree, b->c.level + 1, SPOS_MAX));
 		struct bkey_s_c k = bkey_i_to_s_c(&b->key);
 		ret = bch2_gc_mark_key(trans, btree, b->c.level + 1, NULL, NULL, k, initial);
-		level = b->c.level;
 err_root:
 		bch2_trans_iter_exit(trans, &iter);
 	} while (bch2_err_matches(ret, BCH_ERR_transaction_restart));
-
-	if (ret)
-		return ret;
-
-	for (; level >= target_depth; --level) {
-		struct btree *prev = NULL;
-		struct btree_iter iter;
-		bch2_trans_node_iter_init(trans, &iter, btree, POS_MIN, 0, level,
-					  BTREE_ITER_prefetch);
-
-		ret = for_each_btree_key_continue(trans, iter, 0, k, ({
-			gc_pos_set(c, gc_pos_btree(btree, level, k.k->p));
-			bch2_gc_mark_key(trans, btree, level, &prev, &iter, k, initial);
-		}));
-		if (ret)
-			break;
-	}
-
+err:
+	bch_err_fn(c, ret);
 	return ret;
 }
 

fs/bcachefs/btree_gc.h

@@ -58,6 +58,8 @@ static inline struct gc_pos gc_pos_btree_node(struct btree *b)
 
 static inline int gc_btree_order(enum btree_id btree)
 {
+	if (btree == BTREE_ID_alloc)
+		return -2;
 	if (btree == BTREE_ID_stripes)
 		return -1;
 	return btree;
@@ -65,11 +67,11 @@ static inline int gc_btree_order(enum btree_id btree)
 
 static inline int gc_pos_cmp(struct gc_pos l, struct gc_pos r)
 {
-	return   cmp_int(l.phase, r.phase) ?:
-		 cmp_int(gc_btree_order(l.btree),
-			 gc_btree_order(r.btree)) ?:
-		-cmp_int(l.level, r.level) ?:
-		 bpos_cmp(l.pos, r.pos);
+	return  cmp_int(l.phase, r.phase) ?:
+		cmp_int(gc_btree_order(l.btree),
+			gc_btree_order(r.btree)) ?:
+		cmp_int(l.level, r.level) ?:
+		bpos_cmp(l.pos, r.pos);
 }
 
 static inline bool gc_visited(struct bch_fs *c, struct gc_pos pos)