Re: [PATCH] block: Simplify the bio cloning implementation

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jun 27, 2018 at 04:55:20PM -0700, Bart Van Assche wrote:
> On 06/27/18 16:21, Ming Lei wrote:
> > What we need to do is to only copy the 1st bvec for WRITE_SAME, your patch
> > changes to copy (bio->bi_iter.bi_size / block size) bvecs, then memory corruption
> > may be triggered. So bio_for_each_segment() can't be used here.
> 
> Has it been considered to use memcpy() to copy the bi_vcnt bio_vecs instead
> of using bio_for_each_segment() in bio_clone_bioset()? That will in this
> context probably be even faster than using bio_for_each_segment().

After immutable bvec is introduced:

1) there is little chance in which bvec table need to copy, so performance
may not be a issue, as you see, bio_clone_bioset() is going to die now.

2) bi_vcnt/bi_io_vec can't be used directly on fast-cloned bio

2) code is simplified a lot by using iterator helper since bio can be
advanced in unit of byte. In theory, we may cook a special helper to
speed up the copy, but still depends on use cases.

thanks,
Ming