Re: [PATCH v2 2/9] io_uring: add infra for importing vectored reg buffers

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/4/25 15:40, Pavel Begunkov wrote:
> Add io_import_reg_vec(), which will be responsible for importing
> vectored registered buffers. iovecs are overlapped with the resulting
> bvec in memory, which is why the iovec is expected to be padded in
> iou_vec.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
...
> +int io_import_reg_vec(int ddir, struct iov_iter *iter,
> +			struct io_kiocb *req, struct iou_vec *vec,
> +			unsigned nr_iovs, unsigned iovec_off,
> +			unsigned issue_flags)
> +{
> +	struct io_rsrc_node *node;
> +	struct io_mapped_ubuf *imu;
> +	struct iovec *iov;
> +	unsigned nr_segs;
> +
> +	node = io_find_buf_node(req, issue_flags);
> +	if (!node)
> +		return -EFAULT;
> +	imu = node->buf;
> +	if (imu->is_kbuf)
> +		return -EOPNOTSUPP;
> +	if (!(imu->dir & (1 << ddir)))
> +		return -EFAULT;
> +
> +	iov = vec->iovec + iovec_off;
> +	nr_segs = io_estimate_bvec_size(iov, nr_iovs, imu);

if (sizeof(struct bio_vec) > sizeof(struct iovec)) {
	size_t entry_sz = sizeof(struct iovec);
	size_t bvec_bytes = nr_segs * sizeof(struct bio_vec);
	size_t iovec_off = (bvec_bytes + entry_sz - 1) / entry_sz;

	nr_segs += iovec_off;
}

How about fixing it up like this for now? Instead of overlapping
bvec with iovec, it'd put them back to back and waste some memory
on 32bit.

I can try to make it a bit tighter, remove the if and let
the compiler to optimise it into no-op for x64, or allocate
max(bvec, iovec) * nr and see where it leads. But in either
way IMHO it's better to be left until I get more time.


> +
> +	if (WARN_ON_ONCE(iovec_off + nr_iovs != vec->nr) ||
> +	    nr_segs > vec->nr) {
> +		struct iou_vec tmp_vec = {};
> +		int ret;
> +
> +		ret = io_vec_realloc(&tmp_vec, nr_segs);
> +		if (ret)
> +			return ret;
> +
> +		iovec_off = tmp_vec.nr - nr_iovs;
> +		memcpy(tmp_vec.iovec + iovec_off, iov, sizeof(*iov) * nr_iovs);
> +		io_vec_free(vec);
> +
> +		*vec = tmp_vec;
> +		iov = vec->iovec + iovec_off;
> +		req->flags |= REQ_F_NEED_CLEANUP;
> +	}
> +
> +	return io_vec_fill_bvec(ddir, iter, imu, iov, nr_iovs, vec);
> +}


-- 
Pavel Begunkov