netdev - Re: [PATCH v2 net-next] tcp: fix zerocopy and notsent

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Date:   Tue, 26 Mar 2019 12:30:31 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v2 net-next] tcp: fix zerocopy and notsent_lowat issues

On Tue, Mar 26, 2019 at 11:35 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> My recent patch had at least three problems :
>
> 1) TX zerocopy wants notification when skb is acknowledged,
>    thus we need to call skb_zcopy_clear() if the skb is
>    cached into sk->sk_tx_skb_cache
>
> 2) Some applications might expect precise EPOLLOUT
>    notifications, so we need to update sk->sk_wmem_queued
>    and call sk_mem_uncharge() from sk_wmem_free_skb()
>    in all cases. The SOCK_QUEUE_SHRUNK flag must also be set.
>
> 3) Reuse of saved skb should have used skb_cloned() instead
>   of simply checking if the fast clone has been freed.
>
> Fixes: 472c2e07eef0 ("tcp: add one skb cache for tx")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Cc: Soheil Hassas Yeganeh <soheil@...gle.com>

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

I can't think of other corner cases. Thanks!

> ---
>  include/net/sock.h |  9 +++++----
>  net/ipv4/tcp.c     | 13 +++----------
>  2 files changed, 8 insertions(+), 14 deletions(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 577d91fb56267371c6bc5ae65f7454deba726bd6..7fa2232785226bcafd46b230559964fd16f3c4f4 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1465,13 +1465,14 @@ static inline void sk_mem_uncharge(struct sock *sk, int size)
>
>  static inline void sk_wmem_free_skb(struct sock *sk, struct sk_buff *skb)
>  {
> -       if (!sk->sk_tx_skb_cache) {
> -               sk->sk_tx_skb_cache = skb;
> -               return;
> -       }
>         sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
>         sk->sk_wmem_queued -= skb->truesize;
>         sk_mem_uncharge(sk, skb->truesize);
> +       if (!sk->sk_tx_skb_cache) {
> +               skb_zcopy_clear(skb, true);
> +               sk->sk_tx_skb_cache = skb;
> +               return;
> +       }
>         __kfree_skb(skb);
>  }
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 29b94edf05f9357d3a33744d677827ce624738ae..82bd707c03472f2cebb1a90d5f1c13acc821468f 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -865,14 +865,9 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp,
>  {
>         struct sk_buff *skb;
>
> -       skb = sk->sk_tx_skb_cache;
> -       if (skb && !size) {
> -               const struct sk_buff_fclones *fclones;
> -
> -               fclones = container_of(skb, struct sk_buff_fclones, skb1);
> -               if (refcount_read(&fclones->fclone_ref) == 1) {
> -                       sk->sk_wmem_queued -= skb->truesize;
> -                       sk_mem_uncharge(sk, skb->truesize);
> +       if (likely(!size)) {
> +               skb = sk->sk_tx_skb_cache;
> +               if (skb && !skb_cloned(skb)) {
>                         skb->truesize -= skb->data_len;
>                         sk->sk_tx_skb_cache = NULL;
>                         pskb_trim(skb, 0);
> @@ -2543,8 +2538,6 @@ void tcp_write_queue_purge(struct sock *sk)
>         tcp_rtx_queue_purge(sk);
>         skb = sk->sk_tx_skb_cache;
>         if (skb) {
> -               sk->sk_wmem_queued -= skb->truesize;
> -               sk_mem_uncharge(sk, skb->truesize);
>                 __kfree_skb(skb);
>                 sk->sk_tx_skb_cache = NULL;
>         }
> --
> 2.21.0.392.gf8f6787159e-goog
>