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From: Jan Beulich <jbeulich@suse.com>
To: paul@xen.org, Wei Liu <wl@xen.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Ping: [PATCH] xen-netback: correct success/error reporting for the SKB-with-fraglist case
Date: Thu, 15 Jul 2021 10:58:21 +0200	[thread overview]
Message-ID: <9d27a3eb-1d50-64bb-8785-81de1eef3102@suse.com> (raw)
In-Reply-To: <1a522244-4be8-5e33-77c7-4ea5cf130335@suse.com>

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@suse.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@xen.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 ...

Jan


  reply	other threads:[~2021-07-15  8:58 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-23 16:29 Jan Beulich
2021-02-24 16:39 ` Paul Durrant
2021-02-25  7:33   ` Jan Beulich
2021-02-25 12:11     ` Paul Durrant
2021-02-25 14:00       ` Jan Beulich
2021-02-25 16:23         ` Paul Durrant
2021-05-20 11:46           ` Jan Beulich
2021-07-15  8:58             ` Jan Beulich [this message]
2021-09-16 15:45               ` Ping²: " Jan Beulich
2021-09-16 18:34                 ` Paul Durrant
2021-09-16 21:48                   ` Sander Eikelenboom
2021-09-17  6:21                     ` Jan Beulich

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