netdev - Re: [PATCH bpf v4] xsk: do not discard packet when NETDEV_TX

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Date:   Mon, 14 Sep 2020 09:52:39 -0700
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     magnus.karlsson@...el.com, bjorn.topel@...el.com, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        jonathan.lemon@...il.com, A.Zema@...convsystems.com
Subject: Re: [PATCH bpf v4] xsk: do not discard packet when NETDEV_TX_BUSY

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>

Patch seems to make sense to me, better matches the expectations/path
of the stack.

Reviewed-by: Jesse Brandeburg <jesse.brandeburg@...el.com>