linux-kernel - Re: [PATCH] iov_iter: optimise bvec iov_iter

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Date:   Thu, 3 Dec 2020 09:14:06 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iov_iter: optimise bvec iov_iter_advance()

> @@ -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;
}