netdev - Re: [PATCH v2 net-next 2/2] tcp: fix possible freeze in tx path under memory pressure

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Date:   Tue, 14 Jun 2022 13:42:15 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>, Wei Wang <weiwan@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 net-next 2/2] tcp: fix possible freeze in tx path under
 memory pressure

On Tue, Jun 14, 2022 at 1:17 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> Blamed commit only dealt with applications issuing small writes.
>
> Issue here is that we allow to force memory schedule for the sk_buff
> allocation, but we have no guarantee that sendmsg() is able to
> copy some payload in it.
>
> In this patch, I make sure the socket can use up to tcp_wmem[0] bytes.
>
> For example, if we consider tcp_wmem[0] = 4096 (default on x86),
> and initial skb->truesize being 1280, tcp_sendmsg() is able to
> copy up to 2816 bytes under memory pressure.
>
> Before this patch a sendmsg() sending more than 2816 bytes
> would either block forever (if persistent memory pressure),
> or return -EAGAIN.
>
> For bigger MTU networks, it is advised to increase tcp_wmem[0]
> to avoid sending too small packets.
>
> v2: deal with zero copy paths.
>
> Fixes: 8e4d980ac215 ("tcp: fix behavior for epoll edge trigger")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Very nice find! Thank you!

> ---
>  net/ipv4/tcp.c | 33 +++++++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 14ebb4ec4a51f3c55501aa53423ce897599e8637..56083c2497f0b695c660256aa43f8a743d481697 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -951,6 +951,23 @@ static int tcp_downgrade_zcopy_pure(struct sock *sk, struct sk_buff *skb)
>         return 0;
>  }
>
> +static int tcp_wmem_schedule(struct sock *sk, int copy)
> +{
> +       int left;
> +
> +       if (likely(sk_wmem_schedule(sk, copy)))
> +               return copy;
> +
> +       /* We could be in trouble if we have nothing queued.
> +        * Use whatever is left in sk->sk_forward_alloc and tcp_wmem[0]
> +        * to guarantee some progress.
> +        */
> +       left = sock_net(sk)->ipv4.sysctl_tcp_wmem[0] - sk->sk_wmem_queued;
> +       if (left > 0)
> +               sk_forced_mem_schedule(sk, min(left, copy));
> +       return min(copy, sk->sk_forward_alloc);
> +}
> +
>  static struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
>                                       struct page *page, int offset, size_t *size)
>  {
> @@ -986,7 +1003,11 @@ static struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
>                 tcp_mark_push(tp, skb);
>                 goto new_segment;
>         }
> -       if (tcp_downgrade_zcopy_pure(sk, skb) || !sk_wmem_schedule(sk, copy))
> +       if (tcp_downgrade_zcopy_pure(sk, skb))
> +               return NULL;
> +
> +       copy = tcp_wmem_schedule(sk, copy);
> +       if (!copy)
>                 return NULL;
>
>         if (can_coalesce) {
> @@ -1334,8 +1355,11 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>
>                         copy = min_t(int, copy, pfrag->size - pfrag->offset);
>
> -                       if (tcp_downgrade_zcopy_pure(sk, skb) ||
> -                           !sk_wmem_schedule(sk, copy))
> +                       if (tcp_downgrade_zcopy_pure(sk, skb))
> +                               goto wait_for_space;
> +
> +                       copy = tcp_wmem_schedule(sk, copy);
> +                       if (!copy)
>                                 goto wait_for_space;
>
>                         err = skb_copy_to_page_nocache(sk, &msg->msg_iter, skb,
> @@ -1362,7 +1386,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                                 skb_shinfo(skb)->flags |= SKBFL_PURE_ZEROCOPY;
>
>                         if (!skb_zcopy_pure(skb)) {
> -                               if (!sk_wmem_schedule(sk, copy))
> +                               copy = tcp_wmem_schedule(sk, copy);
> +                               if (!copy)
>                                         goto wait_for_space;
>                         }
>
> --
> 2.36.1.476.g0c4daa206d-goog
>