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Commit 93ab6cc6 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
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tcp: implement mmap() for zero copy receive 



Some networks can make sure TCP payload can exactly fit 4KB pages,
with well chosen MSS/MTU and architectures.

Implement mmap() system call so that applications can avoid
copying data without complex splice() games.

Note that a successful mmap( X bytes) on TCP socket is consuming
bytes, as if recvmsg() has been done. (tp->copied += X)

Only PROT_READ mappings are accepted, as skb page frags
are fundamentally shared and read only.

If tcp_mmap() finds data that is not a full page, or a patch of
urgent data, -EINVAL is returned, no bytes are consumed.

Application must fallback to recvmsg() to read the problematic sequence.

mmap() wont block,  regardless of socket being in blocking or
non-blocking mode. If not enough bytes are in receive queue,
mmap() would return -EAGAIN, or -EIO if socket is in a state
where no other bytes can be added into receive queue.

An application might use SO_RCVLOWAT, poll() and/or ioctl( FIONREAD)
to efficiently use mmap()

On the sender side, MSG_EOR might help to clearly separate unaligned
headers and 4K-aligned chunks if necessary.

Tested:

mlx4 (cx-3) 40Gbit NIC, with tcp_mmap program provided in following patch.
MTU set to 4168  (4096 TCP payload, 40 bytes IPv6 header, 32 bytes TCP header)

Without mmap() (tcp_mmap -s)

received 32768 MB (0 % mmap'ed) in 8.13342 s, 33.7961 Gbit,
  cpu usage user:0.034 sys:3.778, 116.333 usec per MB, 63062 c-switches
received 32768 MB (0 % mmap'ed) in 8.14501 s, 33.748 Gbit,
  cpu usage user:0.029 sys:3.997, 122.864 usec per MB, 61903 c-switches
received 32768 MB (0 % mmap'ed) in 8.11723 s, 33.8635 Gbit,
  cpu usage user:0.048 sys:3.964, 122.437 usec per MB, 62983 c-switches
received 32768 MB (0 % mmap'ed) in 8.39189 s, 32.7552 Gbit,
  cpu usage user:0.038 sys:4.181, 128.754 usec per MB, 55834 c-switches

With mmap() on receiver (tcp_mmap -s -z)

received 32768 MB (100 % mmap'ed) in 8.03083 s, 34.2278 Gbit,
  cpu usage user:0.024 sys:1.466, 45.4712 usec per MB, 65479 c-switches
received 32768 MB (100 % mmap'ed) in 7.98805 s, 34.4111 Gbit,
  cpu usage user:0.026 sys:1.401, 43.5486 usec per MB, 65447 c-switches
received 32768 MB (100 % mmap'ed) in 7.98377 s, 34.4296 Gbit,
  cpu usage user:0.028 sys:1.452, 45.166 usec per MB, 65496 c-switches
received 32768 MB (99.9969 % mmap'ed) in 8.01838 s, 34.281 Gbit,
  cpu usage user:0.02 sys:1.446, 44.7388 usec per MB, 65505 c-switches

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 03f45c88

include/net/tcp.h

+2 −0
Original line number Diff line number Diff line
@@ -404,6 +404,8 @@ int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int nonblock, 
		int flags, int *addr_len);

int tcp_set_rcvlowat(struct sock *sk, int val);

void tcp_data_ready(struct sock *sk);

int tcp_mmap(struct file *file, struct socket *sock,

	     struct vm_area_struct *vma);

void tcp_parse_options(const struct net *net, const struct sk_buff *skb,

		       struct tcp_options_received *opt_rx,

		       int estab, struct tcp_fastopen_cookie *foc);

net/ipv4/af_inet.c

+1 −1
Original line number Diff line number Diff line
@@ -994,7 +994,7 @@ const struct proto_ops inet_stream_ops = { 
	.getsockopt	   = sock_common_getsockopt,

	.sendmsg	   = inet_sendmsg,

	.recvmsg	   = inet_recvmsg,

	.mmap		   = sock_no_mmap,

	.mmap		   = tcp_mmap,

	.sendpage	   = inet_sendpage,

	.splice_read	   = tcp_splice_read,

	.read_sock	   = tcp_read_sock,

net/ipv4/tcp.c

+113 −0
Original line number Diff line number Diff line
@@ -1726,6 +1726,119 @@ int tcp_set_rcvlowat(struct sock *sk, int val) 
}

EXPORT_SYMBOL(tcp_set_rcvlowat);



/* When user wants to mmap X pages, we first need to perform the mapping

 * before freeing any skbs in receive queue, otherwise user would be unable

 * to fallback to standard recvmsg(). This happens if some data in the

 * requested block is not exactly fitting in a page.

 *

 * We only support order-0 pages for the moment.

 * mmap() on TCP is very strict, there is no point

 * trying to accommodate with pathological layouts.

 */

int tcp_mmap(struct file *file, struct socket *sock,

	     struct vm_area_struct *vma)

{

	unsigned long size = vma->vm_end - vma->vm_start;

	unsigned int nr_pages = size >> PAGE_SHIFT;

	struct page **pages_array = NULL;

	u32 seq, len, offset, nr = 0;

	struct sock *sk = sock->sk;

	const skb_frag_t *frags;

	struct tcp_sock *tp;

	struct sk_buff *skb;

	int ret;



	if (vma->vm_pgoff || !nr_pages)

		return -EINVAL;



	if (vma->vm_flags & VM_WRITE)

		return -EPERM;

	/* TODO: Maybe the following is not needed if pages are COW */

	vma->vm_flags &= ~VM_MAYWRITE;



	lock_sock(sk);



	ret = -ENOTCONN;

	if (sk->sk_state == TCP_LISTEN)

		goto out;



	sock_rps_record_flow(sk);



	if (tcp_inq(sk) < size) {

		ret = sock_flag(sk, SOCK_DONE) ? -EIO : -EAGAIN;

		goto out;

	}

	tp = tcp_sk(sk);

	seq = tp->copied_seq;

	/* Abort if urgent data is in the area */

	if (unlikely(tp->urg_data)) {

		u32 urg_offset = tp->urg_seq - seq;



		ret = -EINVAL;

		if (urg_offset < size)

			goto out;

	}

	ret = -ENOMEM;

	pages_array = kvmalloc_array(nr_pages, sizeof(struct page *),

				     GFP_KERNEL);

	if (!pages_array)

		goto out;

	skb = tcp_recv_skb(sk, seq, &offset);

	ret = -EINVAL;

skb_start:

	/* We do not support anything not in page frags */

	offset -= skb_headlen(skb);

	if ((int)offset < 0)

		goto out;

	if (skb_has_frag_list(skb))

		goto out;

	len = skb->data_len - offset;

	frags = skb_shinfo(skb)->frags;

	while (offset) {

		if (frags->size > offset)

			goto out;

		offset -= frags->size;

		frags++;

	}

	while (nr < nr_pages) {

		if (len) {

			if (len < PAGE_SIZE)

				goto out;

			if (frags->size != PAGE_SIZE || frags->page_offset)

				goto out;

			pages_array[nr++] = skb_frag_page(frags);

			frags++;

			len -= PAGE_SIZE;

			seq += PAGE_SIZE;

			continue;

		}

		skb = skb->next;

		offset = seq - TCP_SKB_CB(skb)->seq;

		goto skb_start;

	}

	/* OK, we have a full set of pages ready to be inserted into vma */

	for (nr = 0; nr < nr_pages; nr++) {

		ret = vm_insert_page(vma, vma->vm_start + (nr << PAGE_SHIFT),

				     pages_array[nr]);

		if (ret)

			goto out;

	}

	/* operation is complete, we can 'consume' all skbs */

	tp->copied_seq = seq;

	tcp_rcv_space_adjust(sk);



	/* Clean up data we have read: This will do ACK frames. */

	tcp_recv_skb(sk, seq, &offset);

	tcp_cleanup_rbuf(sk, size);



	ret = 0;

out:

	release_sock(sk);

	kvfree(pages_array);

	return ret;

}

EXPORT_SYMBOL(tcp_mmap);



static void tcp_update_recv_tstamps(struct sk_buff *skb,

				    struct scm_timestamping *tss)

{

net/ipv6/af_inet6.c

+1 −1
Original line number Diff line number Diff line
@@ -579,7 +579,7 @@ const struct proto_ops inet6_stream_ops = { 
	.getsockopt	   = sock_common_getsockopt,	/* ok		*/

	.sendmsg	   = inet_sendmsg,		/* ok		*/

	.recvmsg	   = inet_recvmsg,		/* ok		*/

	.mmap		   = sock_no_mmap,

	.mmap		   = tcp_mmap,

	.sendpage	   = inet_sendpage,

	.sendmsg_locked    = tcp_sendmsg_locked,

	.sendpage_locked   = tcp_sendpage_locked,