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The radix allocator has to be careful to not get lost in recursion trying to allocate metadata blocks for its dirty radix blocks while allocating metadata blocks for others. The first pass had used path data structures to record the references to all the blocks we'd need to modify to reflect the frees and allocations performed while dirtying radix blocks. Once it had all the path blocks it moved the old clean blocks into new dirty locations so that the dirtying couldn't fail. This had two very bad performance implications. First, it meant that trying to read clean versions of dirtied trees would always read the old blocks again because their clean version had been moved to the dirty version. Typically this wouldn't happen but the server does exactly this every time it tries to merge freed blocks back into its avail allocator. This created a significant IO load on the server. Secondly, that block cache move not being allowed to fail motivated us to move to a locked rbtree for the block cache instead of the lockless rcu radix_tree. This changes the recursion avoidance to use per-block private metadata to track every block that we allocate and cow rather than move. Each dirty block knows its parent ref and the blknos it would clear and set. If dirtying fails we can walk back through all the blocks we dirty and restore their original references before dropping all the dirty blocks and returning an error. This lets us get rid of the path structure entirely and results in a much cleaner system. This change meant tracking free blocks without clearing them as they're used to satisfy dirty block allocations. The change now has a cursor that walks the avail metadata tree without modifying it. While building this it became clear that tracking the first set bits of refs doesn't provide any value if we're always searching from a cursor. The cursor ends up providing the same value of avoiding constantly searching empty initial bits and refs. Maintaining the first metadata was just overhead. Signed-off-by: Zach Brown <zab@versity.com>