linux-kernel - Re: [PATCH] nvme-tcp: Check if request has started before processing it

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Message-ID: <b626504c-3427-e8a5-3502-e44a9e79a006@suse.de>
Date:   Mon, 1 Mar 2021 17:53:25 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Daniel Wagner <dwagner@...e.de>, Sagi Grimberg <sagi@...mberg.me>,
        Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme-tcp: Check if request has started before processing
 it

On 3/1/21 5:05 PM, Keith Busch wrote:
> On Mon, Mar 01, 2021 at 02:55:30PM +0100, Hannes Reinecke wrote:
>> On 3/1/21 2:26 PM, Daniel Wagner wrote:
>>> On Sat, Feb 27, 2021 at 02:19:01AM +0900, Keith Busch wrote:
>>>> Crashing is bad, silent data corruption is worse. Is there truly no
>>>> defense against that? If not, why should anyone rely on this?
>>>
>>> If we receive an response for which we don't have a started request, we
>>> know that something is wrong. Couldn't we in just reset the connection
>>> in this case? We don't have to pretend nothing has happened and
>>> continuing normally. This would avoid a host crash and would not create
>>> (more) data corruption. Or I am just too naive?
>>>
>> This is actually a sensible solution.
>> Please send a patch for that.
> 
> Is a bad frame a problem that can be resolved with a reset?
> 
> Even if so, the reset doesn't indicate to the user if previous commands
> completed with bad data, so it still seems unreliable.
> 
We need to distinguish two cases here.
The one is use receiving a frame with an invalid tag, leading to a 
crash. This can be easily resolved by issuing a reset, as clearly the 
command was garbage and we need to invoke error handling (which is reset).

The other case is us receiving a frame with a _duplicate_ tag, ie a tag 
which is _currently_ valid. This is a case which will fail _even now_, 
as we have simply no way of detecting this.

So what again do we miss by fixing the first case?
Apart from a system which does _not_ crash?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
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