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Date: Thu, 6 Apr 2023 20:13:48 -0600
From: David Ahern <dsahern@...nel.org>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
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
Subject: Re: [PATCH net-next v5 05/19] tcp: Support MSG_SPLICE_PAGES
On 4/6/23 7:56 PM, Willem de Bruijn wrote:
> 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?
tcp_sendmsg_locked is already more than 250 lines long and this 47 lines
is compounding it. I was staring at this code 2 weeks ago wondering if
it can be split or refactored to reduce the complexity.
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