Re: [PATCH] block: add bio_iov_iter_nvecs for figuring out nr_vecs

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

On Tue, Dec 01, 2020 at 12:52:51PM +0000, Matthew Wilcox wrote:
> On Tue, Dec 01, 2020 at 08:06:52PM +0800, Ming Lei wrote:
> > Pavel reported that iov_iter_npages is a bit heavy in case of bvec
> > iter.
> > 
> > Turns out it isn't necessary to iterate every page in the bvec iter,
> > and we call iov_iter_npages() just for figuring out how many bio
> > vecs need to be allocated. And we can simply map each vector in bvec iter
> > to bio's vec, so just return iter->nr_segs from bio_iov_iter_nvecs() for
> > bvec iter.
> > 
> > Also rename local variable 'nr_pages' as 'nr_vecs' which exactly matches its
> > real usage.
> > 
> > This patch is based on Mathew's post:
> > 
> > https://lore.kernel.org/linux-block/20201120123931.GN29991@casper.infradead.org/
> 
> But the only reason we want to know 'nr_vecs' is so we can allocate a
> BIO which has that many vecs, right?  But we then don't actually use the
> vecs in the bio because we use the ones already present in the iter.
> That was why I had it return 1, not nr_vecs.
> 
> Did I miss something?

Please see bio_iov_iter_get_pages():

  do {
  		...
      	if (is_bvec)
        	ret = __bio_iov_bvec_add_pages(bio, iter);
		...
   } while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));


So all bvecs in bvec iter will be added to the bio, and it isn't
correct or efficient to just return 1.

thanks,
Ming