Re: [PATCH] iov_iter: optimise bvec iov_iter_advance()

[Christoph Hellwig]

> @@ -1077,6 +1077,20 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
>               i->count -= size;
>               return;
>       }
> +     if (iov_iter_is_bvec(i)) {
> +             struct bvec_iter bi;
> +
> +             bi.bi_size = i->count;
> +             bi.bi_bvec_done = i->iov_offset;
> +             bi.bi_idx = 0;
> +             bvec_iter_advance(i->bvec, &bi, size);
> +
> +             i->bvec += bi.bi_idx;
> +             i->nr_segs -= bi.bi_idx;
> +             i->count = bi.bi_size;
> +             i->iov_offset = bi.bi_bvec_done;
> +             return;
> +     }
>       iterate_and_advance(i, size, v, 0, 0, 0)

I like the idea, but whu not avoid the on-stack bvec_iter and just
open code this entirely using a new helper?  E.g.

static void bio_iov_iter_advance(struct iov_iter *i, size_t bytes)
{
        unsigned int cnt;

        i->count -= bytes;

        bytes += i->iov_offset;
        for (cnt = 0; bytes && bytes >= i->bvec[cnt].bv_len; cnt++)
                bytes -= i->bvec[cnt].bv_len;
        i->iov_offset = bytes;

        i->bvec += cnt;
        i->nr_segs -= cnt;
}