linux-kernel - Re: [PATCH net] xsk: fix __xsk_generic

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Message-ID: <CAJ8uoz12bmCPsr_LFwCDypiwzmH+U7TeLqqykgRhp=8vKX4nQw@mail.gmail.com>
Date: Mon, 24 Feb 2025 18:14:33 +0100
From: Magnus Karlsson <magnus.karlsson@...il.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: Wang Liang <wangliang74@...wei.com>, bjorn@...nel.org, magnus.karlsson@...el.com, 
	maciej.fijalkowski@...el.com, jonathan.lemon@...il.com, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, 
	ast@...nel.org, daniel@...earbox.net, hawk@...nel.org, 
	john.fastabend@...il.com, yuehaibing@...wei.com, zhangchangzhong@...wei.com, 
	netdev@...r.kernel.org, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] xsk: fix __xsk_generic_xmit() error code when cq is full

On Mon, 24 Feb 2025 at 17:00, Stanislav Fomichev <stfomichev@...il.com> wrote:
>
> On 02/24, Magnus Karlsson wrote:
> > On Sat, 22 Feb 2025 at 10:18, Wang Liang <wangliang74@...wei.com> wrote:
> > >
> > > When the cq reservation is failed, the error code is not set which is
> > > initialized to zero in __xsk_generic_xmit(). That means the packet is not
> > > send successfully but sendto() return ok.
> > >
> > > Set the error code and make xskq_prod_reserve_addr()/xskq_prod_reserve()
> > > return values more meaningful when the queue is full.
> >
> > Hi Wang,
> >
> > I agree that this would have been a really good idea if it was
> > implemented from day one, but now I do not dare to change this since
> > it would be changing the uapi. Let us say you have the following quite
> > common code snippet for sending a packet with AF_XDP in skb mode:
> >
> > err = sendmsg();
> > if (err && err != -EAGAIN && err != -EBUSY)
> >     goto die_due_to_error;
> > continue with code
> >
> > This code would with your change go and die suddenly when the
> > completion ring is full instead of working. Maybe there is a piece of
> > code that cleans the completion ring after these lines of code and
> > next time sendmsg() is called, the packet will get sent, so the
> > application used to work.
> >
> > So I say: let us not do this. But if anyone has another opinion, please share.
>
> Can we return -EBUSY from this 'if (xsk_cq_reserve_addr_locked())' case as
> well?

That is a good idea! Though I would return -EAGAIN. When -EBUSY is
returned, the buffer was consumed but not sent. But -EAGAIN means that
the user just has to perform then sendmsg() again and that is exactly
what the user has to do here too.