linux-kernel - Re: [PATCH net-next v2 1/9] xen-netback: Introduce TX grant map definitions

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Date:	Tue, 7 Jan 2014 14:50:59 +0000
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	<ian.campbell@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next v2 1/9] xen-netback: Introduce TX grant map definitions

On 16/12/13 17:50, Wei Liu wrote:
> On Mon, Dec 16, 2013 at 03:21:40PM +0000, Zoltan Kiss wrote:
> [...]
>>>>>>>
>>>>>>> Should this be BUG_ON? AIUI this kthread should be the only one doing
>>>>>>> unmap, right?
>>>>> The NAPI instance can do it as well if it is a small packet fits
>>>>> into PKT_PROT_LEN. But still this scenario shouldn't really happen,
>>>>> I was just not sure we have to crash immediately. Maybe handle it as
>>>>> a fatal error and destroy the vif?
>>>>>
>>> It depends. If this is within the trust boundary, i.e. everything at the
>>> stage should have been sanitized then we should BUG_ON because there's
>>> clearly a bug somewhere in the sanitization process, or in the
>>> interaction of various backend routines.
>>
>> My understanding is that crashing should be avoided if we can bail
>> out somehow. At this point there is clearly a bug in netback
>> somewhere, something unmapped that page before it should have
>> happened, or at least that array get corrupted somehow. However
>> there is a chance that xenvif_fatal_tx_err() can contain the issue,
>> and the rest of the system can go unaffected.
>>
>
> That would make debugging much harder if a crash is caused by a previous
> corrupted array and we pretend we can carry on serving IMHO. Now netback
> is having three routines (NAPI, two kthreads) to serve a single vif, the
> interation among them makes bug hard to reproduce.

OK, I'll make this a BUG() in the next series.

Zoli

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