RE: [RFC PATCH v2 16/48] ip, udp: Support MSG_SPLICE_PAGES

Willem de Bruijn
2023-03-30T14:21:37Z

David Howells wrote:
> Make IP/UDP sendmsg() support MSG_SPLICE_PAGES. This causes pages to be
> spliced from the source iterator if possible (the iterator must be
> ITER_BVEC and the pages must be spliceable).
>
> This allows ->sendpage() to be replaced by something that can handle
> multiple multipage folios in a single transaction.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> cc: "David S. Miller" <davem@davemloft.net>
> cc: Eric Dumazet <edumazet@google.com>
> cc: Jakub Kicinski <kuba@kernel.org>
> cc: Paolo Abeni <pabeni@redhat.com>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: netdev@vger.kernel.org
> ---
> net/ipv4/ip_output.c | 85 +++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 81 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c

A non-RFC version would require the same for ipv6, of course.

> index 4e4e308c3230..07736da70eab 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -973,11 +973,11 @@ static int __ip_append_data(struct sock *sk,
> int hh_len;
> int exthdrlen;
> int mtu;
> - int copy;
> + ssize_t copy;
> int err;
> int offset = 0;
> bool zc = false;
> - unsigned int maxfraglen, fragheaderlen, maxnonfragsize;
> + unsigned int maxfraglen, fragheaderlen, maxnonfragsize, xlength;

Does x here stand for anything?

> int csummode = CHECKSUM_NONE;
> struct rtable *rt = (struct rtable *)cork->dst;
> unsigned int wmem_alloc_delta = 0;
> @@ -1017,6 +1017,7 @@ static int __ip_append_data(struct sock *sk,
> (!exthdrlen || (rt->dst.dev->features & NETIF_F_HW_ESP_TX_CSUM)))
> csummode = CHECKSUM_PARTIAL;
>
> + xlength = length;
> if ((flags & MSG_ZEROCOPY) && length) {
> struct msghdr *msg = from;
>
> @@ -1047,6 +1048,14 @@ static int __ip_append_data(struct sock *sk,
> skb_zcopy_set(skb, uarg, &extra_uref);
> }
> }
> + } else if ((flags & MSG_SPLICE_PAGES) && length) {
> + struct msghdr *msg = from;
> +
> + if (inet->hdrincl)
> + return -EPERM;
> + if (!(rt->dst.dev->features & NETIF_F_SG))
> + return -EOPNOTSUPP;
> + xlength = transhdrlen; /* We need an empty buffer to attach stuff to */
> }
>
> cork->length += length;
> @@ -1074,6 +1083,50 @@ static int __ip_append_data(struct sock *sk,
> unsigned int alloclen, alloc_extra;
> unsigned int pagedlen;
> struct sk_buff *skb_prev;
> +
> + if (unlikely(flags & MSG_SPLICE_PAGES)) {
> + skb_prev = skb;
> + fraggap = skb_prev->len - maxfraglen;
> +
> + alloclen = fragheaderlen + hh_len + fraggap + 15;
> + skb = sock_wmalloc(sk, alloclen, 1, sk->sk_allocation);
> + if (unlikely(!skb)) {
> + err = -ENOBUFS;
> + goto error;
> + }
> +
> + /*
> + * Fill in the control structures
> + */
> + skb->ip_summed = CHECKSUM_NONE;
> + skb->csum = 0;
> + skb_reserve(skb, hh_len);
> +
> + /*
> + * Find where to start putting bytes.
> + */
> + skb_put(skb, fragheaderlen + fraggap);
> + skb_reset_network_header(skb);
> + skb->transport_header = (skb->network_header +
> + fragheaderlen);
> + if (fraggap) {
> + skb->csum = skb_copy_and_csum_bits(
> + skb_prev, maxfraglen,
> + skb_transport_header(skb),
> + fraggap);
> + skb_prev->csum = csum_sub(skb_prev->csum,
> + skb->csum);
> + pskb_trim_unique(skb_prev, maxfraglen);
> + }
> +
> + /*
> + * Put the packet on the pending queue.
> + */
> + __skb_queue_tail(&sk->sk_write_queue, skb);
> + continue;
> + }
> + xlength = length;
> +
> alloc_new_skb:
> skb_prev = skb;
> if (skb_prev)
> @@ -1085,7 +1138,7 @@ static int __ip_append_data(struct sock *sk,
> * If remaining data exceeds the mtu,
> * we know we need more fragment(s).
> */
> - datalen = length + fraggap;
> + datalen = xlength + fraggap;
> if (datalen > mtu - fragheaderlen)
> datalen = maxfraglen - fragheaderlen;
> fraglen = datalen + fragheaderlen;
> @@ -1099,7 +1152,7 @@ static int __ip_append_data(struct sock *sk,
> * because we have no idea what fragment will be
> * the last.
> */
> - if (datalen == length + fraggap)
> + if (datalen == xlength + fraggap)
> alloc_extra += rt->dst.trailer_len;
>
> if ((flags & MSG_MORE) &&
> @@ -1206,6 +1259,30 @@ static int __ip_append_data(struct sock *sk,
> err = -EFAULT;
> goto error;
> }
> + } else if (flags & MSG_SPLICE_PAGES) {
> + struct msghdr *msg = from;
> + struct page *page = NULL, **pages = &page;
> + size_t off;
> +
> + copy = iov_iter_extract_pages(&msg->msg_iter, &pages,
> + copy, 1, 0, &off);
> + if (copy <= 0) {
> + err = copy ?: -EIO;
> + goto error;
> + }
> +
> + err = skb_append_pagefrags(skb, page, off, copy);
> + if (err < 0)
> + goto error;
> +
> + if (skb->ip_summed == CHECKSUM_NONE) {
> + __wsum csum;
> + csum = csum_page(page, off, copy);
> + skb->csum = csum_block_add(skb->csum, csum, skb->len);
> + }
> +
> + skb_len_add(skb, copy);
> + refcount_add(copy, &sk->sk_wmem_alloc);
> } else if (!zc) {
> int i = skb_shinfo(skb)->nr_frags;
>
>

This does add a lot of code to two functions that are already
unwieldy. It may be unavoidable, but it if can use helpers, that would
be preferable.