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

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Date:	Mon, 16 Dec 2013 17:50:36 +0000
From:	Wei Liu <wei.liu2@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>, <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 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.

Wei.
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