otherwise, shrink in-place. as explained in the description of commit
3e16313f8f, the split here is valid
without holding split_merge_lock because all chunks involved are in
the in-use state.
otherwise, shrink in-place. as explained in the description of commit
3e16313f8f, the split here is valid
without holding split_merge_lock because all chunks involved are in
the in-use state.