netdev - [PATCH net] net: do not leave an empty skb in write queue

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Message-ID: <20231019100237.1118733-1-edumazet@google.com>
Date: Thu, 19 Oct 2023 10:02:37 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Soheil Hassas Yeganeh <soheil@...gle.com>, Neal Cardwell <ncardwell@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>, 
	Shakeel Butt <shakeelb@...gle.com>
Subject: [PATCH net] net: do not leave an empty skb in write queue

Under memory stress conditions, tcp_sendmsg_locked()
might call sk_stream_wait_memory(), thus releasing the socket lock.

If a fresh skb has been allocated prior to this,
we should not leave it in the write queue otherwise
tcp_write_xmit() could panic.

This apparently does not happen often, but a future change
in __sk_mem_raise_allocated() that Shakeel and others are
considering would increase chances of being hurt.

Under discussion is to remove this controversial part:

    /* Fail only if socket is _under_ its sndbuf.
     * In this case we cannot block, so that we have to fail.
     */
    if (sk->sk_wmem_queued + size >= sk->sk_sndbuf) {
        /* Force charge with __GFP_NOFAIL */
        if (memcg_charge && !charged) {
            mem_cgroup_charge_skmem(sk->sk_memcg, amt,
                        gfp_memcg_charge() | __GFP_NOFAIL);
        }
        return 1;
    }

Fixes: fdfc5c8594c2 ("tcp: remove empty skb from write queue in error cases")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Shakeel Butt <shakeelb@...gle.com>
---
 net/ipv4/tcp.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index d3456cf840de35b28a6adb682e27d426b0a60f84..a9a49e1d3da11bc6f9ed3baad5dd581400d50c69 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -927,10 +927,11 @@ int tcp_send_mss(struct sock *sk, int *size_goal, int flags)
 	return mss_now;
 }
 
-/* In some cases, both sendmsg() could have added an skb to the write queue,
- * but failed adding payload on it.  We need to remove it to consume less
+/* In some cases, 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.
+ * epoll() users. Another reason is that tcp_write_xmit() does not like
+ * finding an empty skb in the write queue.
  */
 void tcp_remove_empty_skb(struct sock *sk)
 {
@@ -1293,6 +1294,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			tcp_push(sk, flags & ~MSG_MORE, mss_now,
 				 TCP_NAGLE_PUSH, size_goal);
 
+		tcp_remove_empty_skb(sk);
 		err = sk_stream_wait_memory(sk, &timeo);
 		if (err != 0)
 			goto do_error;
-- 
2.42.0.655.g421f12c284-goog