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

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Message-ID: <20200709193431.wruc3u6x5ddnkicv@bsd-mbp.dhcp.thefacebook.com>
Date:   Thu, 9 Jul 2020 12:34:31 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        A.Zema@...convsystems.com
Subject: Re: [PATCH bpf] xsk: do not discard packet when QUEUE_STATE_FROZEN

On Thu, Jul 09, 2020 at 09:30:42PM +0200, Magnus Karlsson wrote:
> On Thu, Jul 9, 2020 at 7:10 PM Magnus Karlsson
> <magnus.karlsson@...il.com> wrote:
> >
> > On Thu, Jul 9, 2020 at 7:06 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
> > >
> > > On Thu, Jul 09, 2020 at 11:45:51AM +0200, Magnus Karlsson wrote:
> > > > In the skb Tx path, transmission of a packet is performed with
> > > > dev_direct_xmit(). When QUEUE_STATE_FROZEN is set in the transmit
> > > > routines, it returns NETDEV_TX_BUSY signifying that it was not
> > > > possible to send the packet 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 and return EAGAIN. As EAGAIN is returned to the application, it
> > > > can then retry the send operation and the packet will finally be sent
> > > > as we will likely not be in the QUEUE_STATE_FROZEN state anymore. So
> > > > 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.
> > >
> > > Doesn't the original code leak the skb if NETDEV_TX_BUSY is returned?
> > > I'm not seeing where it was released.  The new code looks correct.
> >
> > You are correct. Should also have mentioned that in the commit message.
> 
> Jonathan,
> 
> Some context here. The bug report from Arkadiusz started out with the
> unnecessary packet loss. While fixing it, I discovered that it was
> actually leaking memory too. If you want, I can send a v2 that has a
> commit message that mentions both problems? Let me know what you
> prefer.

I think it would be best to mention both problems for the benefit of
future readers.
-- 
Jonathan