netdev - Re: [PATCH V2 net-next 4/7] rds: support for zcopy completion notification

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Date:   Wed, 14 Feb 2018 19:41:49 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, rds-devel@....oracle.com,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>
Subject: Re: [PATCH V2 net-next 4/7] rds: support for zcopy completion notification

On Wed, Feb 14, 2018 at 5:28 AM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> RDS removes a datagram (rds_message) from the retransmit queue when
> an ACK is received. The ACK indicates that the receiver has queued
> the RDS datagram, so that the sender can safely forget the datagram.
> When all references to the rds_message are quiesced, rds_message_purge
> is called to release resources used by the rds_message
>
> If the datagram to be removed had pinned pages set up, add
> an entry to the rs->rs_znotify_queue so that the notifcation
> will be sent up via rds_rm_zerocopy_callback() when the
> rds_message is eventually freed by rds_message_purge.
>
> rds_rm_zerocopy_callback() attempts to batch the number of cookies
> sent with each notification  to a max of SO_EE_ORIGIN_MAX_ZCOOKIES.
> This is achieved by checking the tail skb in the sk_error_queue:
> if this has room for one more cookie, the cookie from the
> current notification is added; else a new skb is added to the
> sk_error_queue. Every invocation of rds_rm_zerocopy_callback() will
> trigger a ->sk_error_report to notify the application.
>
> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
> ---

> +static void rds_rm_zerocopy_callback(struct rds_sock *rs,
> +                                    struct rds_znotifier *znotif)
> +{
> +       struct sock *sk = rds_rs_to_sk(rs);
> +       struct sk_buff *skb, *tail;
> +       struct sock_exterr_skb *serr;
> +       unsigned long flags;
> +       struct sk_buff_head *q;
> +       u32 cookie = znotif->z_cookie;
> +
> +       q = &sk->sk_error_queue;
> +       spin_lock_irqsave(&q->lock, flags);
> +       tail = skb_peek_tail(q);
> +
> +       if (tail && skb_zcookie_add(tail, cookie)) {
> +               spin_unlock_irqrestore(&q->lock, flags);
> +               mm_unaccount_pinned_pages(&znotif->z_mmp);
> +               consume_skb(rds_skb_from_znotifier(znotif));
> +               sk->sk_error_report(sk);
> +               return;
> +       }
> +
> +       skb = rds_skb_from_znotifier(znotif);
> +       serr = SKB_EXT_ERR(skb);
> +       serr->ee.ee_errno = 0;
> +       serr->ee.ee_origin = SO_EE_ORIGIN_ZCOOKIE;
> +       serr->ee.ee_info = 0;
> +       serr->ee.ee_code |= SO_EE_CODE_ZEROCOPY_COPIED;

One more thing: this code notifies that the operation succeeded, but
the data was copied in the process. It does not have to be set otherwise.