netdev - Re: [PATCH bpf v5] xsk: do not discard packet when NETDEV_TX

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Date:   Wed, 16 Sep 2020 23:41:03 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Magnus Karlsson <magnus.karlsson@...il.com>,
        magnus.karlsson@...el.com, bjorn.topel@...el.com, ast@...nel.org,
        netdev@...r.kernel.org, jonathan.lemon@...il.com
Cc:     A.Zema@...convsystems.com
Subject: Re: [PATCH bpf v5] xsk: do not discard packet when NETDEV_TX_BUSY

On 9/16/20 2:00 PM, Magnus Karlsson wrote:
> From: Magnus Karlsson <magnus.karlsson@...el.com>
> 
> In the skb Tx path, transmission of a packet is performed with
> dev_direct_xmit(). When NETDEV_TX_BUSY is set in the drivers, it
> signifies that it was not possible to send the packet right now,
> please try later. Unfortunately, the xsk transmit code discarded the
> packet and returned EBUSY to the application. Fix this unnecessary
> packet loss, by not discarding the packet in the Tx ring and return
> EAGAIN. As EAGAIN is returned to the application, it can then retry
> the send operation later and the packet will then likely be sent as
> the driver will then likely have space/resources to send the packet.
> 
> In summary, EAGAIN tells the application that the packet was not
> discarded from the Tx ring and that it needs to call send()
> again. EBUSY, on the other hand, signifies that the packet was not
> sent and discarded from the Tx ring. The application needs to put the
> packet on the Tx ring again if it wants it to be sent.
> 
> Fixes: 35fcde7f8deb ("xsk: support for Tx")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
> Reported-by: Arkadiusz Zema <A.Zema@...convsystems.com>
> Suggested-by: Arkadiusz Zema <A.Zema@...convsystems.com>
> Suggested-by: Daniel Borkmann <daniel@...earbox.net>

Hopefully patchwork/vger recovers soon, but looks good & I've applied
this one in meantime (also kept Jesse's prior Reviewed-by given there
were no fundamental changes). Thanks!