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: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 113 insertions, 0 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index c768d306b657..438fbca96cd3 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -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)
 {