Re: [PATCH net-next v7 03/16] net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES

[Paolo Abeni <pabeni@redhat.com>]

On Mon, 2023-05-15 at 10:33 +0100, David Howells wrote:
> Add a function to handle MSG_SPLICE_PAGES being passed internally to
> sendmsg().  Pages are spliced into the given socket buffer if possible and
> copied in if not (e.g. they're slab pages or have a zero refcount).
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Eric Dumazet <edumazet@google.com>
> cc: "David S. Miller" <davem@davemloft.net>
> cc: David Ahern <dsahern@kernel.org>
> cc: Jakub Kicinski <kuba@kernel.org>
> cc: Paolo Abeni <pabeni@redhat.com>
> cc: Al Viro <viro@zeniv.linux.org.uk>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: netdev@vger.kernel.org
> ---
> 
> Notes:
>     ver #7)
>      - Export function.
>      - Never copy data, return -EIO if sendpage_ok() returns false.
> 
>  include/linux/skbuff.h |  3 ++
>  net/core/skbuff.c      | 95 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 98 insertions(+)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 4c0ad48e38ca..1c5f0ac6f8c3 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -5097,5 +5097,8 @@ static inline void skb_mark_for_recycle(struct sk_buff *skb)
>  #endif
>  }
>  
> +ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
> +			     ssize_t maxsize, gfp_t gfp);
> +
>  #endif	/* __KERNEL__ */
>  #endif	/* _LINUX_SKBUFF_H */
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 7f53dcb26ad3..56d629ea2f3d 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6892,3 +6892,98 @@ 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
> + * 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)
> +{
> +	struct page *pages[8], **ppages = pages;
> +	unsigned int i;
> +	ssize_t spliced = 0, ret = 0;
> +	size_t frag_limit = READ_ONCE(sysctl_max_skb_frags);

Minor nit: please respect the reverse x-mas tree order (there are a few
other occurrences around)

> +
> +	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;
> +		}
> +
> +		if (space == 0 &&
> +		    !skb_can_coalesce(skb, skb_shinfo(skb)->nr_frags,
> +				      pages[0], off)) {
> +			iov_iter_revert(iter, len);
> +			break;
> +		}

It looks like the above condition/checks duplicate what the later
skb_append_pagefrags() will perform below. I guess the above chunk
could be removed?

> +
> +		i = 0;
> +		do {
> +			struct page *page = pages[i++];
> +			size_t part = min_t(size_t, PAGE_SIZE - off, len);
> +
> +			ret = -EIO;
> +			if (!sendpage_ok(page))
> +				goto out;

My (limited) understanding is that the current sendpage code assumes
that the caller provides/uses pages suitable for such use. The existing
sendpage_ok() check is in place as way to try to catch possible code
bug - via the WARN_ONCE().

I think the same could be done here?

Thanks!

Paolo

> +
> +			ret = skb_append_pagefrags(skb, page, off, part,
> +						   frag_limit);
> +			if (ret < 0) {
> +				iov_iter_revert(iter, len);
> +				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);
>