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

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Message-ID: <df5b1deb-7d72-4f52-86c2-959ea4dffad4@huawei.com>
Date: Tue, 25 Feb 2025 09:45:12 +0800
From: Wang Liang <wangliang74@...wei.com>
To: Magnus Karlsson <magnus.karlsson@...il.com>, Stanislav Fomichev
	<stfomichev@...il.com>
CC: <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


在 2025/2/25 1:14, Magnus Karlsson 写道:
> 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.


Thank you for the suggestion!
Changing the uapi is indeed a high-risk act. Return -EAGAIN is a much 
better choice.
The cq is full usually because it is not released in time, try to send 
msg again is appropriate.
I will send a new patch later, and look forward to getting more advice. 
Thanks.