On 06/27/18 17:30, Ming Lei wrote:

One core idea of immutable bvec is to use bio->bi_iter and the original
bvec table to iterate over anywhere in the bio. That is why .bi_io_vec
needs to copy, but not see any reason why .bi_vcnt needs to do.

Do you have use cases on .bi_vcnt for cloned bio?

So far this is only a theoretical concern. There are many functions in 
the block layer that use .bi_vcnt, and it is a lot of work to figure out 
all the callers of all these functions.

Bart.