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@xxxxxxxxxxxxxxxxxxxx/ > > 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
