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

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Date:   Fri, 17 Sep 2021 08:21:51 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Sander Eikelenboom <sander@...elenboom.it>
Cc:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        paul@....org, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Wei Liu <wl@....org>
Subject: Re: Ping²: [PATCH] xen-netback: correct success/error reporting for the SKB-with-fraglist case

On 16.09.2021 23:48, Sander Eikelenboom wrote:
> On 16/09/2021 20:34, Paul Durrant wrote:
>> 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
> 
> Could it be the missing "net" or "net-next" designation in the subject 
> [1] which seems to be used and important within their patchwork 
> semi-automated workflow ?

I wouldn't exclude this, but having to play special games there means
I'll try to refrain from fixing any bugs under net/ in the future. I'll
resend following their apparently required pattern.

Jan