When reading received messages with MSG_PEEK, we sometines have to read
the leading bytes of the stream several times, only to reach the bytes
we really want. This is clearly non-optimal.
What we would want is something similar to pread/preadv(), but working
even for tcp sockets. At the same time, we obviously don't want to add
any new arguments to the recv/recvmsg() calls.
In this commit, we allow the user to set iovec.iov_base in the first
vector entry to NULL. This tells the socket to skip the first entry,
hence making the iov_len field of that entry indicate the offset value.
This way, there is no need to add any new arguments.
This change is simple and non-intrusive, and should be safe addition to
the socket API. We have measured it to give a throughput improvement of
8-10 % for the protocol splicer 'passst', which is used in KubeVirt
containers.
Signed-off-by: Jon Maloy <jmaloy(a)redhat.com>
works with original msghdr
Signed-off-by: Jon Maloy <jmaloy(a)redhat.com>
---
net/ipv4/tcp.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 33f559f491c8..1d89337e89b6 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2428,6 +2428,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg,
size_t len,
struct tcp_sock *tp = tcp_sk(sk);
int copied = 0;
u32 peek_seq;
+ u32 peek_offset;
u32 *seq;
unsigned long used;
int err;
@@ -2435,7 +2436,6 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg,
size_t len,
long timeo;
struct sk_buff *skb, *last;
u32 urg_hole = 0;
-
err = -ENOTCONN;
if (sk->sk_state == TCP_LISTEN)
goto out;
@@ -2469,6 +2469,14 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg,
size_t len,
if (flags & MSG_PEEK) {
peek_seq = tp->copied_seq;
seq = &peek_seq;
+ if (msg->msg_iter.iov[0].iov_base == NULL) {
+ peek_offset = msg->msg_iter.iov[0].iov_len;
+ msg->msg_iter.iov = &msg->msg_iter.iov[1];
+ msg->msg_iter.nr_segs -= 1;
+ msg->msg_iter.count -= peek_offset;
+ len -= peek_offset;
+ *seq += peek_offset;
+ }
}
target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);
--
2.39.0