Re: [PATCH V11 03/19] block: introduce bio_for_each_bvec()

[Christoph Hellwig]

On Wed, Nov 21, 2018 at 11:31:36PM +0800, Ming Lei wrote:
> > But while looking over this I wonder why we even need the max_seg_len
> > here.  The only thing __bvec_iter_advance does it to move bi_bvec_done
> > and bi_idx forward, with corresponding decrements of bi_size.  As far
> > as I can tell the only thing that max_seg_len does is that we need
> > to more iterations of the while loop to archive the same thing.
> > 
> > And actual bvec used by the caller will be obtained using
> > bvec_iter_bvec or segment_iter_bvec depending on if they want multi-page
> > or single-page variants.
> 
> Right, we let __bvec_iter_advance() serve for both multi-page and single-page
> case, then we have to tell it via one way or another, now we use the constant
> of 'max_seg_len'.
> 
> Or you suggest to implement two versions of __bvec_iter_advance()?

No - I think we can always use the code without any segment in
bvec_iter_advance.  Because bvec_iter_advance only operates on the
iteractor, the generation of an actual single-page or multi-page
bvec is left to the caller using the bvec_iter_bvec or segment_iter_bvec
helpers.  The only difference is how many bytes you can move the
iterator forward in a single loop iteration - so if you pass in
PAGE_SIZE as the max_seg_len you just will have to loop more often
for a large enough bytes, but not actually do anything different.