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    Subject[RFC PATCH v2 16/48] ip, udp: Support MSG_SPLICE_PAGES
    Date
    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
    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;
    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;

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