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

[Matthew Wilcox <willy@infradead.org>]

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?

> +static inline int bio_iov_iter_nvecs(const struct iov_iter *i, int maxvecs)
> +{
> +	if (!iov_iter_count(i))
> +		return 0;
> +	if (iov_iter_is_bvec(i))
> +               return min_t(int, maxvecs, i->nr_segs);
> +	return iov_iter_npages(i, maxvecs);
> +}