radix_tree_next_chunk: redundant search for next slot in hole

[Probir Roy]

Hi,

I am running Phoronix-fio benchmark on Linux kernel 4.18.0-rc5. I
observe that radix_tree_next_chunk search shows some form of a
redundant walk while called from radix_tree_for_each_slot iterator.
While searching for next slot in a hole, it walks through the same
slots over n over.

void __rcu **radix_tree_next_chunk(const struct radix_tree_root *root,
                 struct radix_tree_iter *iter, unsigned flags)
{
...
      if ((flags & RADIX_TREE_ITER_TAGGED) ?
                !tag_get(node, tag, offset) : !child) {
         ...
          while (++offset < RADIX_TREE_MAP_SIZE) {
                    void *slot = rcu_dereference_raw(   /* redundant
slot walk */
                            node->slots[offset]);
                    if (slot)
                        break;
                }
        }
...
}

Can you please explain such behavior? Any feedback/recommendation
about this case is very welcome.



-- 
Probir Roy