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Date: Thu, 6 Apr 2023 21:56:19 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: David Howells <dhowells@...hat.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Matthew Wilcox <willy@...radead.org>,
Al Viro <viro@...iv.linux.org.uk>,
Christoph Hellwig <hch@...radead.org>,
Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
Christian Brauner <brauner@...nel.org>,
Chuck Lever III <chuck.lever@...cle.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH net-next v5 05/19] tcp: Support MSG_SPLICE_PAGES
On Thu, Apr 6, 2023 at 5:43 AM David Howells <dhowells@...hat.com> wrote:
>
> Make TCP's sendmsg() support MSG_SPLICE_PAGES. This causes pages to be
> spliced from the source iterator.
>
> This allows ->sendpage() to be replaced by something that can handle
> multiple multipage folios in a single transaction.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Eric Dumazet <edumazet@...gle.com>
> cc: "David S. Miller" <davem@...emloft.net>
> cc: David Ahern <dsahern@...nel.org>
> cc: Jakub Kicinski <kuba@...nel.org>
> cc: Paolo Abeni <pabeni@...hat.com>
> cc: Jens Axboe <axboe@...nel.dk>
> cc: Matthew Wilcox <willy@...radead.org>
> cc: netdev@...r.kernel.org
> ---
> net/ipv4/tcp.c | 67 ++++++++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 60 insertions(+), 7 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index fd68d49490f2..510bacc7ce7b 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1221,7 +1221,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> int flags, err, copied = 0;
> int mss_now = 0, size_goal, copied_syn = 0;
> int process_backlog = 0;
> - bool zc = false;
> + int zc = 0;
> long timeo;
>
> flags = msg->msg_flags;
> @@ -1232,17 +1232,22 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> if (msg->msg_ubuf) {
> uarg = msg->msg_ubuf;
> net_zcopy_get(uarg);
> - zc = sk->sk_route_caps & NETIF_F_SG;
> + if (sk->sk_route_caps & NETIF_F_SG)
> + zc = 1;
> } else if (sock_flag(sk, SOCK_ZEROCOPY)) {
> uarg = msg_zerocopy_realloc(sk, size, skb_zcopy(skb));
> if (!uarg) {
> err = -ENOBUFS;
> goto out_err;
> }
> - zc = sk->sk_route_caps & NETIF_F_SG;
> - if (!zc)
> + if (sk->sk_route_caps & NETIF_F_SG)
> + zc = 1;
> + else
> uarg_to_msgzc(uarg)->zerocopy = 0;
> }
> + } else if (unlikely(msg->msg_flags & MSG_SPLICE_PAGES) && size) {
> + if (sk->sk_route_caps & NETIF_F_SG)
> + zc = 2;
> }
>
> if (unlikely(flags & MSG_FASTOPEN || inet_sk(sk)->defer_connect) &&
> @@ -1305,7 +1310,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> goto do_error;
>
> while (msg_data_left(msg)) {
> - int copy = 0;
> + ssize_t copy = 0;
>
> skb = tcp_write_queue_tail(sk);
> if (skb)
> @@ -1346,7 +1351,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> if (copy > msg_data_left(msg))
> copy = msg_data_left(msg);
>
> - if (!zc) {
> + if (zc == 0) {
> bool merge = true;
> int i = skb_shinfo(skb)->nr_frags;
> struct page_frag *pfrag = sk_page_frag(sk);
> @@ -1391,7 +1396,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> page_ref_inc(pfrag->page);
> }
> pfrag->offset += copy;
> - } else {
> + } else if (zc == 1) {
Instead of 1 and 2, MSG_ZEROCOPY and MSG_SPLICE_PAGES make the code
more self-documenting.
> /* First append to a fragless skb builds initial
> * pure zerocopy skb
> */
> @@ -1412,6 +1417,54 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> if (err < 0)
> goto do_error;
> copy = err;
> + } else if (zc == 2) {
> + /* Splice in data. */
> + struct page *page = NULL, **pages = &page;
> + size_t off = 0, part;
> + bool can_coalesce;
> + int i = skb_shinfo(skb)->nr_frags;
> +
> + copy = iov_iter_extract_pages(&msg->msg_iter, &pages,
> + copy, 1, 0, &off);
> + if (copy <= 0) {
> + err = copy ?: -EIO;
> + goto do_error;
> + }
> +
> + can_coalesce = skb_can_coalesce(skb, i, page, off);
> + if (!can_coalesce && i >= READ_ONCE(sysctl_max_skb_frags)) {
> + tcp_mark_push(tp, skb);
> + iov_iter_revert(&msg->msg_iter, copy);
> + goto new_segment;
> + }
> + if (tcp_downgrade_zcopy_pure(sk, skb)) {
> + iov_iter_revert(&msg->msg_iter, copy);
> + goto wait_for_space;
> + }
> +
> + part = tcp_wmem_schedule(sk, copy);
> + iov_iter_revert(&msg->msg_iter, copy - part);
> + if (!part)
> + goto wait_for_space;
> + copy = part;
> +
> + if (can_coalesce) {
> + skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
> + } else {
> + get_page(page);
> + skb_fill_page_desc_noacc(skb, i, page, off, copy);
> + }
> + page = NULL;
> +
> + if (!(flags & MSG_NO_SHARED_FRAGS))
> + skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
> +
> + skb->len += copy;
> + skb->data_len += copy;
> + skb->truesize += copy;
> + sk_wmem_queued_add(sk, copy);
> + sk_mem_charge(sk, copy);
> +
Similar to udp, perhaps in a helper?
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