blist_next_leaf_alloc walks over all the meta-nodes between one leaf and the next one, and if blocks are allocated from the next leaf, it walks back toward where it started, as long as there are interleaving meta-nodes to be updated on account of the last free blocks under those meta-nodes being allocated. Only if the walk goes all the way back to the starting point must we calculate the position of the meta-node that is the least-comment parent of one leaf and the next, and update a bit in that meta-node to indicate the allocation of its last free block.
There's no need to start calculating the position of that least-common parent until the walk back reaches the original starting point, and there's no need for a calculation that updates 'radius' to tell us when we've walked back to the beginning, since comparing scan to next suffices for that.