linux-kernel - Re: [PATCH v2] nvme-tcp: Check if request has started before processing it

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Message-ID: <3156c563-94a4-4278-3835-b1f56f71869a@grimberg.me>
Date:   Fri, 7 May 2021 13:26:11 -0700
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Hannes Reinecke <hare@...e.de>, Keith Busch <kbusch@...nel.org>
Cc:     "Ewan D. Milne" <emilne@...hat.com>,
        Daniel Wagner <dwagner@...e.de>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] nvme-tcp: Check if request has started before
 processing it


>>>> 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...