Re: [PATCH net-next v10 03/16] net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2023/5/22 20:11, David Howells wrote:

Hi, David

I am not very familiar with the 'struct iov_iter' yet, just two
questions below.

> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 7f53dcb26ad3..f4a5b51aed22 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6892,3 +6892,91 @@ nodefer:	__kfree_skb(skb);
>  	if (unlikely(kick) && !cmpxchg(&sd->defer_ipi_scheduled, 0, 1))
>  		smp_call_function_single_async(cpu, &sd->defer_csd);
>  }
> +
> +static void skb_splice_csum_page(struct sk_buff *skb, struct page *page,
> +				 size_t offset, size_t len)
> +{
> +	const char *kaddr;
> +	__wsum csum;
> +
> +	kaddr = kmap_local_page(page);
> +	csum = csum_partial(kaddr + offset, len, 0);
> +	kunmap_local(kaddr);
> +	skb->csum = csum_block_add(skb->csum, csum, skb->len);
> +}
> +
> +/**
> + * skb_splice_from_iter - Splice (or copy) pages to skbuff
> + * @skb: The buffer to add pages to
> + * @iter: Iterator representing the pages to be added
> + * @maxsize: Maximum amount of pages to be added
> + * @gfp: Allocation flags
> + *
> + * This is a common helper function for supporting MSG_SPLICE_PAGES.  It
> + * extracts pages from an iterator and adds them to the socket buffer if
> + * possible, copying them to fragments if not possible (such as if they're slab
> + * pages).
> + *
> + * Returns the amount of data spliced/copied or -EMSGSIZE if there's

I am not seeing any copying done directly in the skb_splice_from_iter(),
maybe iov_iter_extract_pages() has done copying for it?

> + * insufficient space in the buffer to transfer anything.
> + */
> +ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
> +			     ssize_t maxsize, gfp_t gfp)
> +{
> +	size_t frag_limit = READ_ONCE(sysctl_max_skb_frags);
> +	struct page *pages[8], **ppages = pages;
> +	ssize_t spliced = 0, ret = 0;
> +	unsigned int i;
> +
> +	while (iter->count > 0) {
> +		ssize_t space, nr;
> +		size_t off, len;
> +
> +		ret = -EMSGSIZE;
> +		space = frag_limit - skb_shinfo(skb)->nr_frags;
> +		if (space < 0)
> +			break;
> +
> +		/* We might be able to coalesce without increasing nr_frags */
> +		nr = clamp_t(size_t, space, 1, ARRAY_SIZE(pages));
> +
> +		len = iov_iter_extract_pages(iter, &ppages, maxsize, nr, 0, &off);
> +		if (len <= 0) {
> +			ret = len ?: -EIO;
> +			break;
> +		}
> +
> +		i = 0;
> +		do {
> +			struct page *page = pages[i++];
> +			size_t part = min_t(size_t, PAGE_SIZE - off, len);
> +
> +			ret = -EIO;
> +			if (WARN_ON_ONCE(!sendpage_ok(page)))
> +				goto out;
> +
> +			ret = skb_append_pagefrags(skb, page, off, part,
> +						   frag_limit);
> +			if (ret < 0) {
> +				iov_iter_revert(iter, len);

I am not sure I understand the error handling here, doesn't 'len'
indicate the remaining size of the data to be appended to skb, maybe
we should revert the size of data that is already appended to skb here?
Does 'spliced' need to be adjusted accordingly?

> +				goto out;
> +			}
> +
> +			if (skb->ip_summed == CHECKSUM_NONE)
> +				skb_splice_csum_page(skb, page, off, part);
> +
> +			off = 0;
> +			spliced += part;
> +			maxsize -= part;
> +			len -= part;
> +		} while (len > 0);
> +
> +		if (maxsize <= 0)
> +			break;
> +	}
> +
> +out:
> +	skb_len_add(skb, spliced);
> +	return spliced ?: ret;
> +}
> +EXPORT_SYMBOL(skb_splice_from_iter);
> 
> 
> .
>