Hi,
After bio splitting is introduced, the splitted bio can be fast-cloned,
which is correct because biovecs has become immutable since v3.13.
Unfortunately bio_will_gap() isn't ready for this kind of change,
because it figures out the last bvec via 'bi_io_vec[prev->bi_vcnt - 1]'
directly.
It is observed that lots of BIOs are merges even the virt boundary
limit is violated, and the issue is reported from Sagi Grimberg.