linux-kernel - Re: [PATCH v8 13/13] nvmet-passthru: support block accounting

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Date:   Mon, 9 Sep 2019 16:15:25 -0700
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:     Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Stephen Bates <sbates@...thlin.com>, Jens Axboe <axboe@...com>,
        Keith Busch <kbusch@...nel.org>,
        Max Gurtovoy <maxg@...lanox.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v8 13/13] nvmet-passthru: support block accounting


>>>>> Support block disk accounting by setting the RQF_IO_STAT flag
>>>>> and gendisk in the request.
>>>>>
>>>>> After this change, IO counts will be reflected correctly in
>>>>> /proc/diskstats for drives being used by passthru.
>>>>>
>>>>> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
>>>>> ---
>>>>>     drivers/nvme/target/io-cmd-passthru.c | 5 ++++-
>>>>>     1 file changed, 4 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/nvme/target/io-cmd-passthru.c b/drivers/nvme/target/io-cmd-passthru.c
>>>>> index 7557927a3451..63f12750a80d 100644
>>>>> --- a/drivers/nvme/target/io-cmd-passthru.c
>>>>> +++ b/drivers/nvme/target/io-cmd-passthru.c
>>>>> @@ -410,6 +410,9 @@ static struct request *nvmet_passthru_blk_make_request(struct nvmet_req *req,
>>>>>     	if (unlikely(IS_ERR(rq)))
>>>>>     		return rq;
>>>>>     
>>>>> +	if (blk_queue_io_stat(q) && cmd->common.opcode != nvme_cmd_flush)
>>>>> +		rq->rq_flags |= RQF_IO_STAT;
>>>
>>> Thanks for the review!
>>>
>>>> Does flush has data bytes in the request? Why the special casing?
>>>
>>> Well it was special cased in the vanilla blk account flow... But I think
>>> it's required to be special cased so the IO and in_flight counts don't
>>> count flushes (as they do not for regular block device traffic).
>>
>> I think that the accounting exclude I/O that is yielded from the flush
>> sequence. Don't think its relevant here...
> 
> What? Per blk_account_io_done(), RQF_FLUSH_SEQ will not be set by us for
> passthru commands and I don't think it's appropriate to do so. Thus, if
> we set RQF_IO_STAT for passthru flush commands, they will be counted
> which we do not want.

Have you considered to have nvmet_passthru_blk_make_request set RQF_FUA
for nvme_cmd_flush? this way blk_insert_flush will be called and
RQF_FLUSH_SEQ will be set and you don't need to worry about this
special casing...