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

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Date:   Thu, 25 Feb 2021 16:23:26 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Wei Liu <wl@....org>
Subject: Re: [PATCH] xen-netback: correct success/error reporting for the
 SKB-with-fraglist case

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>

> Jan
>