[PATCH for-next v7 4/5] block: add helper to map bvec iterator for passthrough

[Christoph Hellwig]

> -static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
> +static struct bio *bio_map_get(struct request *rq, unsigned int nr_vecs,
>  		gfp_t gfp_mask)

bio_map_get is a very confusing name.  And I also still think this is
the wrong way to go.  If plain slab allocations don't use proper
per-cpu caches we have a MM problem and need to talk to the slab
maintainers and not use the overkill bio_set here.

> +/* Prepare bio for passthrough IO given an existing bvec iter */
> +int blk_rq_map_user_bvec(struct request *rq, struct iov_iter *iter)

I'm a little confused about the interface we're trying to present from
the block layer to the driver here.

blk_rq_map_user_iov really should be able to detect that it is called
on a bvec iter and just do the right thing rather than needing different
helpers.

> +		/*
> +		 * If the queue doesn't support SG gaps and adding this
> +		 * offset would create a gap, disallow it.
> +		 */
> +		if (bvprvp && bvec_gap_to_prev(lim, bvprvp, bv->bv_offset))
> +			goto out_err;

So now you limit the input that is accepted?  That's not really how
iov_iters are used.   We can either try to reshuffle the bvecs, or
just fall back to the copy data version as blk_rq_map_user_iov does
for 'weird' iters˙

> +
> +		/* check full condition */
> +		if (nsegs >= nr_segs || bytes > UINT_MAX - bv->bv_len)
> +			goto out_err;
> +
> +		if (bytes + bv->bv_len <= nr_iter &&
> +				bv->bv_offset + bv->bv_len <= PAGE_SIZE) {
> +			nsegs++;
> +			bytes += bv->bv_len;
> +		} else
> +			goto out_err;

Nit: This would read much better as:

		if (bytes + bv->bv_len > nr_iter)
			goto out_err;
		if (bv->bv_offset + bv->bv_len > PAGE_SIZE)
			goto out_err;

		bytes += bv->bv_len;
		nsegs++;