netdev - Re: Ping²: [PATCH] xen-netback: correct success/error reporting for the SKB-with-fraglist case

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Message-ID: <0dff83ff-629a-7179-9fef-77bd1fbf3d09@xen.org>
Date:   Thu, 16 Sep 2021 19:34:19 +0100
From:   Paul Durrant <xadimgnik@...il.com>
To:     Jan Beulich <jbeulich@...e.com>, Wei Liu <wl@....org>
Cc:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Ping²: [PATCH] xen-netback: correct success/error reporting for the SKB-with-fraglist case

On 16/09/2021 16:45, Jan Beulich wrote:
> On 15.07.2021 10:58, Jan Beulich wrote:
>> On 20.05.2021 13:46, Jan Beulich wrote:
>>> On 25.02.2021 17:23, Paul Durrant wrote:
>>>> On 25/02/2021 14:00, Jan Beulich wrote:
>>>>> On 25.02.2021 13:11, Paul Durrant wrote:
>>>>>> On 25/02/2021 07:33, Jan Beulich wrote:
>>>>>>> On 24.02.2021 17:39, Paul Durrant wrote:
>>>>>>>> On 23/02/2021 16:29, Jan Beulich wrote:
>>>>>>>>> When re-entering the main loop of xenvif_tx_check_gop() a 2nd time, the
>>>>>>>>> special considerations for the head of the SKB no longer apply. Don't
>>>>>>>>> mistakenly report ERROR to the frontend for the first entry in the list,
>>>>>>>>> even if - from all I can tell - this shouldn't matter much as the overall
>>>>>>>>> transmit will need to be considered failed anyway.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>>>>>>>>>
>>>>>>>>> --- a/drivers/net/xen-netback/netback.c
>>>>>>>>> +++ b/drivers/net/xen-netback/netback.c
>>>>>>>>> @@ -499,7 +499,7 @@ check_frags:
>>>>>>>>>      				 * the header's copy failed, and they are
>>>>>>>>>      				 * sharing a slot, send an error
>>>>>>>>>      				 */
>>>>>>>>> -				if (i == 0 && sharedslot)
>>>>>>>>> +				if (i == 0 && !first_shinfo && sharedslot)
>>>>>>>>>      					xenvif_idx_release(queue, pending_idx,
>>>>>>>>>      							   XEN_NETIF_RSP_ERROR);
>>>>>>>>>      				else
>>>>>>>>>
>>>>>>>>
>>>>>>>> I think this will DTRT, but to my mind it would make more sense to clear
>>>>>>>> 'sharedslot' before the 'goto check_frags' at the bottom of the function.
>>>>>>>
>>>>>>> That was my initial idea as well, but
>>>>>>> - I think it is for a reason that the variable is "const".
>>>>>>> - There is another use of it which would then instead need further
>>>>>>>      amending (and which I believe is at least part of the reason for
>>>>>>>      the variable to be "const").
>>>>>>>
>>>>>>
>>>>>> Oh, yes. But now that I look again, don't you want:
>>>>>>
>>>>>> if (i == 0 && first_shinfo && sharedslot)
>>>>>>
>>>>>> ? (i.e no '!')
>>>>>>
>>>>>> The comment states that the error should be indicated when the first
>>>>>> frag contains the header in the case that the map succeeded but the
>>>>>> prior copy from the same ref failed. This can only possibly be the case
>>>>>> if this is the 'first_shinfo'
>>>>>
>>>>> I don't think so, no - there's a difference between "first frag"
>>>>> (at which point first_shinfo is NULL) and first frag list entry
>>>>> (at which point first_shinfo is non-NULL).
>>>>
>>>> Yes, I realise I got it backwards. Confusing name but the comment above
>>>> its declaration does explain.
>>>>
>>>>>
>>>>>> (which is why I still think it is safe to unconst 'sharedslot' and
>>>>>> clear it).
>>>>>
>>>>> And "no" here as well - this piece of code
>>>>>
>>>>> 		/* First error: if the header haven't shared a slot with the
>>>>> 		 * first frag, release it as well.
>>>>> 		 */
>>>>> 		if (!sharedslot)
>>>>> 			xenvif_idx_release(queue,
>>>>> 					   XENVIF_TX_CB(skb)->pending_idx,
>>>>> 					   XEN_NETIF_RSP_OKAY);
>>>>>
>>>>> specifically requires sharedslot to have the value that was
>>>>> assigned to it at the start of the function (this property
>>>>> doesn't go away when switching from fragments to frag list).
>>>>> Note also how it uses XENVIF_TX_CB(skb)->pending_idx, i.e. the
>>>>> value the local variable pending_idx was set from at the start
>>>>> of the function.
>>>>>
>>>>
>>>> True, we do have to deal with freeing up the header if the first map
>>>> error comes on the frag list.
>>>>
>>>> Reviewed-by: Paul Durrant <paul@....org>
>>>
>>> Since I've not seen this go into 5.13-rc, may I ask what the disposition
>>> of this is?
>>
>> I can't seem to spot this in 5.14-rc either. I have to admit I'm
>> increasingly puzzled ...
> 
> Another two months (and another release) later and still nothing. Am
> I doing something wrong? Am I wrongly assuming that maintainers would
> push such changes up the chain?
> 

It has my R-b so it ought to go in via netdev AFAICT.

   Paul