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Date:	Mon, 24 Nov 2014 00:02:45 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	David Miller <davem@...emloft.net>, torvalds@...ux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	target-devel@...r.kernel.org, nab@...ux-iscsi.org,
	hch@...radead.org
Subject: Re: [PATCH 07/17] new helpers: skb_copy_datagram_from_iter() and
 zerocopy_sg_from_iter()

On Sat, 2014-11-22 at 04:33 +0000, Al Viro wrote:
[...]
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -572,6 +572,77 @@ fault:
>  }
>  EXPORT_SYMBOL(skb_copy_datagram_from_iovec);
>  

Missing kernel-doc.

> +int skb_copy_datagram_from_iter(struct sk_buff *skb, int offset,
> +				 struct iov_iter *from,
> +				 int len)
> +{
> +	int start = skb_headlen(skb);
> +	int i, copy = start - offset;
> +	struct sk_buff *frag_iter;
> +
> +	/* Copy header. */
> +	if (copy > 0) {
> +		if (copy > len)
> +			copy = len;
> +		if (copy_from_iter(skb->data + offset, copy, from) != copy)
> +			goto fault;
> +		if ((len -= copy) == 0)
> +			return 0;
> +		offset += copy;
> +	}
> +
> +	/* Copy paged appendix. Hmm... why does this look so complicated? */
> +	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> +		int end;
> +		const skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
> +
> +		WARN_ON(start > offset + len);
> +
> +		end = start + skb_frag_size(frag);
> +		if ((copy = end - offset) > 0) {
> +			size_t copied;

Blank line needed after a declaration.

> +			if (copy > len)
> +				copy = len;
> +			copied = copy_page_from_iter(skb_frag_page(frag),
> +					  frag->page_offset + offset - start,
> +					  copy, from);
> +			if (copied != copy)
> +				goto fault;
> +
> +			if (!(len -= copy))
> +				return 0;

The other two instances of this condition are written as:

			if ((len -= copy) == 0)

Similarly in skb_copy_bits().

> +			offset += copy;
> +		}
> +		start = end;
> +	}
> +
> +	skb_walk_frags(skb, frag_iter) {
> +		int end;
> +
> +		WARN_ON(start > offset + len);
> +
> +		end = start + frag_iter->len;
> +		if ((copy = end - offset) > 0) {
> +			if (copy > len)
> +				copy = len;
> +			if (skb_copy_datagram_from_iter(frag_iter,
> +							offset - start,
> +							from, copy))
> +				goto fault;
> +			if ((len -= copy) == 0)
> +				return 0;
> +			offset += copy;
> +		}
> +		start = end;
> +	}
> +	if (!len)
> +		return 0;
> +
> +fault:
> +	return -EFAULT;
> +}
> +EXPORT_SYMBOL(skb_copy_datagram_from_iter);
> +
>  /**
>   *	zerocopy_sg_from_iovec - Build a zerocopy datagram from an iovec
>   *	@skb: buffer to copy
> @@ -643,6 +714,50 @@ int zerocopy_sg_from_iovec(struct sk_buff *skb, const struct iovec *from,
>  }
>  EXPORT_SYMBOL(zerocopy_sg_from_iovec);
>  

Missing kernel-doc.

> +int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *from)
> +{
> +	int len = iov_iter_count(from);
> +	int copy = min_t(int, skb_headlen(skb), len);
> +	int i = 0;
> +
> +	/* copy up to skb headlen */
> +	if (skb_copy_datagram_from_iter(skb, 0, from, copy))
> +		return -EFAULT;
> +
> +	while (iov_iter_count(from)) {
> +		struct page *pages[MAX_SKB_FRAGS];
> +		size_t start;
> +		ssize_t copied;
> +		unsigned long truesize;
> +		int n = 0;
> +
> +		copied = iov_iter_get_pages(from, pages, ~0U, MAX_SKB_FRAGS, &start);
> +		if (copied < 0)
> +			return -EFAULT;
> +
> +		truesize = DIV_ROUND_UP(copied + start, PAGE_SIZE) * PAGE_SIZE;

PAGE_ALIGN(copied + start) ?

> +		skb->data_len += copied;
> +		skb->len += copied;
> +		skb->truesize += truesize;
> +		atomic_add(truesize, &skb->sk->sk_wmem_alloc);
> +		while (copied) {
> +			int off = start;

This variable seems redundant.  Can't we use start directly and move the
'start = 0' to the bottom of the loop?

> +			int size = min_t(int, copied, PAGE_SIZE - off);
> +			start = 0;
> +			if (i < MAX_SKB_FRAGS)
> +				skb_fill_page_desc(skb, i, pages[n], off, size);
> +			else
> +				put_page(pages[n]);

Why is this condition needed, given we told iov_iter_get_pages() to
limit to MAX_SKB_FRAGS pages?

> +			copied -= size;
> +			i++, n++;
> +		}
> +		if (i > MAX_SKB_FRAGS)
> +			return -EMSGSIZE;

Same here.

Ben.

> +	}
> +	return 0;
> +}
> +EXPORT_SYMBOL(zerocopy_sg_from_iter);
> +
>  static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
>  				      u8 __user *to, int len,
>  				      __wsum *csump)

-- 
Ben Hutchings
Never put off till tomorrow what you can avoid all together.

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