Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v2] nvme-tcp: Check if request has started before
 processing it
To:     Hannes Reinecke <hare@suse.de>, Keith Busch <kbusch@kernel.org>
Cc:     "Ewan D. Milne" <emilne@redhat.com>,
        Daniel Wagner <dwagner@suse.de>,
        linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
        Jens Axboe <axboe@fb.com>, Christoph Hellwig <hch@lst.de>
References: <20210301175601.116405-1-dwagner@suse.de>
 <6b51a989-5551-e243-abda-5872411ec3ff@grimberg.me>
 <20210311094345.ogm2lxqfuszktuhp@beryllium.lan>
 <70af5b02-10c1-ab0b-1dfc-5906216871b4@grimberg.me>
 <2fc7a320c86f75507584453dd2fbd744de5c170d.camel@redhat.com>
 <ed3ccac0-79ed-fe10-89eb-d403820b4c6a@grimberg.me>
 <20210330232813.GA1935968@dhcp-10-100-145-180.wdc.com>
 <756aef10-e693-276f-82ac-514a2832b07f@grimberg.me>
 <492b8393-fc35-f58a-3768-94632a083c93@suse.de>
From:   Sagi Grimberg <sagi@grimberg.me>
Message-ID: <3156c563-94a4-4278-3835-b1f56f71869a@grimberg.me>
Date:   Fri, 7 May 2021 13:26:11 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.7.1
MIME-Version: 1.0
In-Reply-To: <492b8393-fc35-f58a-3768-94632a083c93@suse.de>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk


>>>> I would also mention, that a crash is not even the scariest issue that
>>>> we can see here, because if the request happened to be reused we are
>>>> in the silent data corruption realm...
>>>
>>> If this does happen, I think we have to come up with some way to
>>> mitigate it. We're not utilizing the full 16 bits of the command_id, so
>>> maybe we can append something like a generation sequence number that can
>>> be checked for validity.
>>
>> That's actually a great idea. scsi needs unique tags so it encodes the
>> hwq in the upper 16 bits giving the actual tag the lower 16 bits which
>> is more than enough for a single queue. We can do the same with
>> a gencnt that will increment in both submission and completion and we
>> can validate against it.
>>
>> This will be useful for all transports, so maintaining it in
>> nvme_req(rq)->genctr and introducing a helper like:
>> rq = nvme_find_tag(tagset, cqe->command_id)
>> That will filter genctr, locate the request.
>>
>> Also:
>> nvme_validate_request_gen(rq, cqe->command_id) that would
>> compare against it.
>>
>>
>> And then a helper to set the command_id like:
>> cmd->common.command_id = nvme_request_command_id(rq)
>> that will both increment the genctr and build a command_id
>> from it.
>>
>> Thoughts?
>>
> 
> Well, that would require a modification to the CQE specification, no?
> fmds was not amused when I proposed that :-(

Why would that require a modification to the CQE? it's just using say
4 msbits of the command_id to a running sequence...