概述
sendmsg系统调用在tcp层的实现是tcp_sendmsg函数，该函数完成以下任务：从用户空间读取数据，拷贝到内核skb，将skb加入到发送队列的任务，调用发送函数；函数在执行过程中会锁定控制块，避免软中断在tcp层的影响；函数核心流程为，在发送数据时，查看是否能够将数据合并到发送队列中最后一个skb中，如果不能合并，则新申请一个skb；拷贝过程中，如果skb的线性区域有空间，则优先使用线性区域，线性区域空间不足，则使用分页区域；拷贝完成后，调用发送函数发送数据；
代码分析
int tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
struct sockcm_cookie sockc;
int flags, err, copied = ;
int mss_now = , size_goal, copied_syn = ;
bool process_backlog = false;
bool sg;
long timeo; /* 加锁，避免与软中断的冲突 */
lock_sock(sk); /* 获取标记 */
flags = msg->msg_flags; /* fastopen和defer */
if (unlikely(flags & MSG_FASTOPEN || inet_sk(sk)->defer_connect)) {
err = tcp_sendmsg_fastopen(sk, msg, &copied_syn, size);
if (err == -EINPROGRESS && copied_syn > )
goto out;
else if (err)
goto out_err;
} /* 获取阻塞时间，非阻塞为0 */
timeo = sock_sndtimeo(sk, flags & MSG_DONTWAIT); /* 限速检查 */
tcp_rate_check_app_limited(sk); /* is sending application-limited? */ /* Wait for a connection to finish. One exception is TCP Fast Open
* (passive side) where data is allowed to be sent before a connection
* is fully established.
*/
/* 等待连接完成状态，fastopen的被动打开方例外 */
if ((( << sk->sk_state) & ~(TCPF_ESTABLISHED | TCPF_CLOSE_WAIT)) &&
!tcp_passive_fastopen(sk)) {
/* 等待连接完成 */
err = sk_stream_wait_connect(sk, &timeo);
if (err != )
goto do_error;
} if (unlikely(tp->repair)) {
if (tp->repair_queue == TCP_RECV_QUEUE) {
copied = tcp_send_rcvq(sk, msg, size);
goto out_nopush;
} err = -EINVAL;
if (tp->repair_queue == TCP_NO_QUEUE)
goto out_err; /* 'common' sending to sendq */
} sockc.tsflags = sk->sk_tsflags;
if (msg->msg_controllen) {
err = sock_cmsg_send(sk, msg, &sockc);
if (unlikely(err)) {
err = -EINVAL;
goto out_err;
}
} /* This should be in poll */
/* 清除异步队列已满标记 */
sk_clear_bit(SOCKWQ_ASYNC_NOSPACE, sk); /* Ok commence sending. */
copied = ; restart:
/* 获取mss,gso情况下size_goal记录总mss=页数*mss */
mss_now = tcp_send_mss(sk, &size_goal, flags); err = -EPIPE;
/* 已经关闭了发送端 */
if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))
goto do_error; /* 网卡分散聚合标记 */
sg = !!(sk->sk_route_caps & NETIF_F_SG); /* 遍历发送缓存 */
while (msg_data_left(msg)) {
int copy = ;
int max = size_goal; /* 拿到发送队列的尾部skb */
skb = tcp_write_queue_tail(sk);
/* 有skb未发送 */
if (tcp_send_head(sk)) {
/* 网卡不支持校验和，调整最大长度 */
if (skb->ip_summed == CHECKSUM_NONE)
max = mss_now;
/* 该skb还能够容纳的数据长度 */
copy = max - skb->len;
} /* 剩余空间为0，或者不能合并，分配一个新的skb */
if (copy <= || !tcp_skb_can_collapse_to(skb)) {
bool first_skb; new_segment:
/* Allocate new segment. If the interface is SG,
* allocate skb fitting to single page.
*/
/* 空闲内存不足，进入等待 */
if (!sk_stream_memory_free(sk))
goto wait_for_sndbuf; if (process_backlog && sk_flush_backlog(sk)) {
process_backlog = false;
goto restart;
}
first_skb = skb_queue_empty(&sk->sk_write_queue); /* 分配skb */
skb = sk_stream_alloc_skb(sk,
select_size(sk, sg, first_skb),
sk->sk_allocation,
first_skb);
/* 分配失败，等待 */
if (!skb)
goto wait_for_memory; process_backlog = true;
/*
* Check whether we can use HW checksum.
*/
/* 网卡允许计算校验和 */
if (sk_check_csum_caps(sk))
skb->ip_summed = CHECKSUM_PARTIAL; /* 添加到发送队列 */
skb_entail(sk, skb);
copy = size_goal;
max = size_goal; /* All packets are restored as if they have
* already been sent. skb_mstamp isn't set to
* avoid wrong rtt estimation.
*/
if (tp->repair)
TCP_SKB_CB(skb)->sacked |= TCPCB_REPAIRED;
} /* Try to append data to the end of skb. */
/* 拷贝的数据不能超过实际数据块的长度 */
if (copy > msg_data_left(msg))
copy = msg_data_left(msg); /* Where to copy to? */
/* 线性区域还有空间 */
if (skb_availroom(skb) > ) {
/* We have some space in skb head. Superb! */
/* 取要拷贝的数量和线性区域的较小值 */
copy = min_t(int, copy, skb_availroom(skb));
/* 从用户空间拷贝到内核 */
err = skb_add_data_nocache(sk, skb, &msg->msg_iter, copy);
if (err)
goto do_fault;
}
/* 线性区域没有空间，则使用分页区 */
else {
bool merge = true;
/* 获取页数量 */
int i = skb_shinfo(skb)->nr_frags;
/* 获取缓存的页 */
struct page_frag *pfrag = sk_page_frag(sk); /* 检查是否有足够的空间，空间不足则申请新页,失败则等待 */
if (!sk_page_frag_refill(sk, pfrag))
goto wait_for_memory; /* 不能合并 */
if (!skb_can_coalesce(skb, i, pfrag->page,
pfrag->offset)) {
/* 页数量超过限制 || 网卡不支持分散聚合*/
if (i >= sysctl_max_skb_frags || !sg) {
/* 增加push标记，尽快处理数据 */
tcp_mark_push(tp, skb);
/* 新申请一个skb */
goto new_segment;
}
/* 不合并 */
merge = false;
} /* 获取能够合并的空间 */
copy = min_t(int, copy, pfrag->size - pfrag->offset); /* 发送合法性检查 */
if (!sk_wmem_schedule(sk, copy))
goto wait_for_memory; /* 拷贝数据到分页 */
err = skb_copy_to_page_nocache(sk, &msg->msg_iter, skb,
pfrag->page,
pfrag->offset,
copy);
if (err)
goto do_error; /* Update the skb. */
if (merge) {
/* 合并的，需要增加数据量 */
skb_frag_size_add(&skb_shinfo(skb)->frags[i - ], copy);
} else {
/* 非合并的，对新页进行初始化 */
skb_fill_page_desc(skb, i, pfrag->page,
pfrag->offset, copy);
page_ref_inc(pfrag->page);
} /* 记录新的偏移 */
pfrag->offset += copy;
} /* 第一次拷贝，清除push标记 */
if (!copied)
TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_PSH; /* 更新发送队列的最后一个序号 */
tp->write_seq += copy;
/* 更新skb结束序号 */
TCP_SKB_CB(skb)->end_seq += copy;
tcp_skb_pcount_set(skb, ); /* 更新拷贝数量 */
copied += copy; /* 数据都拷贝完成 */
if (!msg_data_left(msg)) {
if (unlikely(flags & MSG_EOR))
TCP_SKB_CB(skb)->eor = ;
goto out;
} /* 还能继续拷贝数据 带外数据 修复模式，继续拷贝 */
if (skb->len < max || (flags & MSG_OOB) || unlikely(tp->repair))
continue; /* 需要使用push标记 */
if (forced_push(tp)) {
/* 打psh标记 */
tcp_mark_push(tp, skb);
/* 发送队列中的多个skb */
__tcp_push_pending_frames(sk, mss_now, TCP_NAGLE_PUSH);
}
/* 否则，有数据要发送，则发送一个skb */
else if (skb == tcp_send_head(sk))
tcp_push_one(sk, mss_now);
continue; wait_for_sndbuf:
/* 设置空间不足标记 */
set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
wait_for_memory:
/* 已经有拷贝数据到发送队列，则发送之 */
if (copied)
tcp_push(sk, flags & ~MSG_MORE, mss_now,
TCP_NAGLE_PUSH, size_goal); /* 等待内存足够 */
err = sk_stream_wait_memory(sk, &timeo);
if (err != )
goto do_error; /* 重新计算mss */
mss_now = tcp_send_mss(sk, &size_goal, flags);
} out:
/* 已经拷贝数据到发送队列，则发送之 */
if (copied) {
tcp_tx_timestamp(sk, sockc.tsflags, tcp_write_queue_tail(sk));
tcp_push(sk, flags, mss_now, tp->nonagle, size_goal);
}
out_nopush:
release_sock(sk);
return copied + copied_syn; do_fault:
/* skb中没有数据，从发送队列中删除skb */
if (!skb->len) {
tcp_unlink_write_queue(skb, sk);
/* It is the one place in all of TCP, except connection
* reset, where we can be unlinking the send_head.
*/
tcp_check_send_head(sk, skb);
sk_wmem_free_skb(sk, skb);
} do_error:
if (copied + copied_syn)
goto out;
out_err:
err = sk_stream_error(sk, flags, err);
/* make sure we wake any epoll edge trigger waiter */
if (unlikely(skb_queue_len(&sk->sk_write_queue) == &&
err == -EAGAIN)) {
sk->sk_write_space(sk);
tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED);
}
release_sock(sk);
return err;
}