bcachefs: bch2_bkey_get_empty_slot()

Add a new helper for allocating a new slot in a btree.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_update.h

@@ -86,6 +86,9 @@ int bch2_btree_node_update_key_get_iter(struct btree_trans *,
 int bch2_trans_update_extent(struct btree_trans *, struct btree_iter *,
 			     struct bkey_i *, enum btree_update_flags);
 
+int bch2_bkey_get_empty_slot(struct btree_trans *, struct btree_iter *,
+			     enum btree_id, struct bpos);
+
 int __must_check bch2_trans_update(struct btree_trans *, struct btree_iter *,
 				   struct bkey_i *, enum btree_update_flags);
 int __must_check bch2_trans_update_buffered(struct btree_trans *,

fs/bcachefs/btree_update_leaf.c

@@ -1735,6 +1735,37 @@ int __must_check bch2_trans_update_buffered(struct btree_trans *trans,
 	return 0;
 }
 
+int bch2_bkey_get_empty_slot(struct btree_trans *trans, struct btree_iter *iter,
+			     enum btree_id btree, struct bpos end)
+{
+	struct bkey_s_c k;
+	int ret = 0;
+
+	bch2_trans_iter_init(trans, iter, btree, POS_MAX, BTREE_ITER_INTENT);
+	k = bch2_btree_iter_prev(iter);
+	ret = bkey_err(k);
+	if (ret)
+		goto err;
+
+	bch2_btree_iter_advance(iter);
+	k = bch2_btree_iter_peek_slot(iter);
+	ret = bkey_err(k);
+	if (ret)
+		goto err;
+
+	BUG_ON(k.k->type != KEY_TYPE_deleted);
+
+	if (bkey_gt(k.k->p, end)) {
+		ret = -BCH_ERR_ENOSPC_btree_slot;
+		goto err;
+	}
+
+	return 0;
+err:
+	bch2_trans_iter_exit(trans, iter);
+	return ret;
+}
+
 void bch2_trans_commit_hook(struct btree_trans *trans,
 			    struct btree_trans_commit_hook *h)
 {

fs/bcachefs/errcode.h

@@ -92,6 +92,7 @@
 	x(ENOSPC,			ENOSPC_sb_replicas)			\
 	x(ENOSPC,			ENOSPC_sb_members)			\
 	x(ENOSPC,			ENOSPC_sb_crypt)			\
+	x(ENOSPC,			ENOSPC_btree_slot)			\
 	x(0,				open_buckets_empty)			\
 	x(0,				freelist_empty)				\
 	x(BCH_ERR_freelist_empty,	no_buckets_found)			\

fs/bcachefs/subvolume.c

@@ -909,32 +909,19 @@ int bch2_subvolume_create(struct btree_trans *trans, u64 inode,
 			  u32 *new_snapshotid,
 			  bool ro)
 {
-	struct bch_fs *c = trans->c;
 	struct btree_iter dst_iter, src_iter = (struct btree_iter) { NULL };
 	struct bkey_i_subvolume *new_subvol = NULL;
 	struct bkey_i_subvolume *src_subvol = NULL;
-	struct bkey_s_c k;
 	u32 parent = 0, new_nodes[2], snapshot_subvols[2];
 	int ret = 0;
 
-	for_each_btree_key(trans, dst_iter, BTREE_ID_subvolumes, SUBVOL_POS_MIN,
-			   BTREE_ITER_SLOTS|BTREE_ITER_INTENT, k, ret) {
-		if (bkey_gt(k.k->p, SUBVOL_POS_MAX))
-			break;
-
-		/*
-		 * bch2_subvolume_delete() doesn't flush the btree key cache -
-		 * ideally it would but that's tricky
-		 */
-		if (bkey_deleted(k.k) &&
-		    !bch2_btree_key_cache_find(c, BTREE_ID_subvolumes, dst_iter.pos))
-			goto found_slot;
-	}
-
-	if (!ret)
+	ret = bch2_bkey_get_empty_slot(trans, &dst_iter,
+				BTREE_ID_subvolumes, POS(0, U32_MAX));
+	if (ret == -BCH_ERR_ENOSPC_btree_slot)
 		ret = -BCH_ERR_ENOSPC_subvolume_create;
-	goto err;
-found_slot:
+	if (ret)
+		return ret;
+
 	snapshot_subvols[0] = dst_iter.pos.offset;
 	snapshot_subvols[1] = src_subvolid;