Re: [PATCH net] tcp: remove empty skb from write queue in error cases

[Jason Baron <jbaron@akamai.com>]

On 8/26/19 12:19 PM, Eric Dumazet wrote:
> Vladimir Rutsky reported stuck TCP sessions after memory pressure
> events. Edge Trigger epoll() user would never receive an EPOLLOUT
> notification allowing them to retry a sendmsg().
> 
> Jason tested the case of sk_stream_alloc_skb() returning NULL,
> but there are other paths that could lead both sendmsg() and sendpage()
> to return -1 (EAGAIN), with an empty skb queued on the write queue.
> 
> This patch makes sure we remove this empty skb so that
> Jason code can detect that the queue is empty, and
> call sk->sk_write_space(sk) accordingly.
> 

Makes sense, thanks. I think this check for empty queue could also
benefit from and update to use tcp_write_queue_empty(). I will send a
follow-up for that.

Thanks,

-Jason



> Fixes: ce5ec440994b ("tcp: ensure epoll edge trigger wakeup when write queue is empty")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Jason Baron <jbaron@akamai.com>
> Reported-by: Vladimir Rutsky <rutsky@google.com>
> Cc: Soheil Hassas Yeganeh <soheil@google.com>
> Cc: Neal Cardwell <ncardwell@google.com>
> ---
>  net/ipv4/tcp.c | 30 ++++++++++++++++++++----------
>  1 file changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 77b485d60b9d0e00edc4e2f0d6c5bb3a9460b23b..61082065b26a068975c411b74eb46739ab0632ca 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -935,6 +935,22 @@ static int tcp_send_mss(struct sock *sk, int *size_goal, int flags)
>  	return mss_now;
>  }
>  
> +/* In some cases, both sendpage() and sendmsg() could have added
> + * an skb to the write queue, but failed adding payload on it.
> + * We need to remove it to consume less memory, but more
> + * importantly be able to generate EPOLLOUT for Edge Trigger epoll()
> + * users.
> + */
> +static void tcp_remove_empty_skb(struct sock *sk, struct sk_buff *skb)
> +{
> +	if (skb && !skb->len) {
> +		tcp_unlink_write_queue(skb, sk);
> +		if (tcp_write_queue_empty(sk))
> +			tcp_chrono_stop(sk, TCP_CHRONO_BUSY);
> +		sk_wmem_free_skb(sk, skb);
> +	}
> +}
> +
>  ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
>  			 size_t size, int flags)
>  {
> @@ -1064,6 +1080,7 @@ ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
>  	return copied;
>  
>  do_error:
> +	tcp_remove_empty_skb(sk, tcp_write_queue_tail(sk));
>  	if (copied)
>  		goto out;
>  out_err:
> @@ -1388,18 +1405,11 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>  	sock_zerocopy_put(uarg);
>  	return copied + copied_syn;
>  
> +do_error:
> +	skb = tcp_write_queue_tail(sk);
>  do_fault:
> -	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.
> -		 */
> -		if (tcp_write_queue_empty(sk))
> -			tcp_chrono_stop(sk, TCP_CHRONO_BUSY);
> -		sk_wmem_free_skb(sk, skb);
> -	}
> +	tcp_remove_empty_skb(sk, skb);
>  
> -do_error:
>  	if (copied + copied_syn)
>  		goto out;
>  out_err:
>