[PATCH V10 09/19] block: introduce bio_bvecs()

[Sagi Grimberg]

>> Wait, I see that the bvec is still a single array per bio. When you said
>> a table I thought you meant a 2-dimentional array...
> 
> I mean a new 1-d table A has to be created for multiple bios in one rq,
> and build it in the following way
> 
>             rq_for_each_bvec(tmp, rq, rq_iter)
>                      *A = tmp;
> 
> Then you can pass A to iov_iter_bvec() & send().
> 
> Given it is over TCP, I guess it should be doable for you to preallocate one
> 256-bvec table in one page for each request, then sets the max segment size as
> (unsigned int)-1, and max segment number as 256, the preallocated table
> should work anytime.

256 bvec table is really a lot to preallocate, especially when its not
needed, I can easily initialize the bvec_iter on the bio bvec. If this
involves preallocation of the worst-case than I don't consider this to
be an improvement.