Re: [PATCH 1/3] iov_iter: add helper to save iov_iter state

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Sep 10, 2021 at 12:25:34PM -0600, Jens Axboe wrote:
> +void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state,
> +		      ssize_t did_bytes)
> +{
> +	if (WARN_ON_ONCE(!iov_iter_is_bvec(i) && !iter_is_iovec(i)) &&
> +			 !iov_iter_is_kvec(i))
> +		return;
> +	i->iov_offset = state->iov_offset;
> +	i->count = state->count;
> +	/*
> +	 * For the *vec iters, nr_segs + iov is constant - if we increment
> +	 * the vec, then we also decrement the nr_segs count. Hence we don't
> +	 * need to track both of these, just one is enough and we can deduct
> +	 * the other from that. ITER_{BVEC,IOVEC,KVEC} all have their pointers
> +	 * unionized, so we don't need to handle them individually.
> +	 */
> +	i->iov -= state->nr_segs - i->nr_segs;
> +	i->nr_segs = state->nr_segs;
> +}

No.  You can have iovec and kvec handled jointly (struct iovec and struct kvec
always have the same size).  You can *not* do that to bvec - check sizeof of
struct bio_vec and struct iovec on 32bit targets.