linux-kernel - Re: [PATCH V2 7/8] nvme: use blk_mq_queue_tag_inflight

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Date:   Wed, 27 Mar 2019 10:03:26 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        James Smart <jsmart2021@...il.com>,
        Bart Van Assche <bvanassche@....org>,
        Ming Lei <tom.leiming@...il.com>,
        Josef Bacik <josef@...icpanda.com>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Busch, Keith" <keith.busch@...el.com>,
        Hannes Reinecke <hare@...e.de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>
Subject: Re: [PATCH V2 7/8] nvme: use blk_mq_queue_tag_inflight_iter

Hi Keith

On 3/27/19 7:57 AM, Keith Busch wrote:
> On Mon, Mar 25, 2019 at 08:05:53PM -0700, jianchao.wang wrote:
>> What if there used to be a io scheduler and leave some stale requests of sched tags ?
>> Or the nr_hw_queues was decreased and leave the hctx->fq->flush_rq ?
> 
> Requests internally queued in scheduler or block layer are not eligible
> for the nvme driver's iterator callback. We only use it to reclaim
> dispatched requests that the target can't return, which only applies to
> requests that must have a valid rq->tag value from hctx->tags.
>  
>> The stable request could be some tings freed and used
>> by others and the state field happen to be overwritten to non-zero...
> 
> I am not sure I follow what this means. At least for nvme, every queue
> sharing the same tagset is quiesced and frozen, there should be no
> request state in flux at the time we iterate.
> 

In nvme_dev_disable, when we try to reclaim the in-flight requests with blk_mq_tagset_busy_iter,
the request_queues are quiesced but just start-freeze.
We will try to _drain_ the in-flight requests for the _shutdown_ case when controller is not dead.
For the reset case, there still could be someone escapes the checking of queue freezing and enters
blk_mq_make_request and tries to allocate tag, then we may get,

generic_make_request        nvme_dev_disable
 -> blk_queue_enter              
                              -> nvme_start_freeze (just start freeze, no drain)
                              -> nvme_stop_queues
 -> blk_mq_make_request
  - > blk_mq_get_request      -> blk_mq_tagset_busy_iter
     -> blk_mq_get_tag
                      　　        -> bt_tags_for_each
                         　　　       -> bt_tags_iter
                              　　       -> rq = tags->rqs[] ---> [1]
     -> blk_mq_rq_ctx_init
       -> data->hctx->tags->rqs[rq->tag] = rq;

The rq got on position [1] could be a stale request that has been freed due to,
1. a hctx->fq.flush_rq of dead request_queue that shares the same tagset
2. a removed io scheduler's sched request

And this stale request may have been used by others and the request->state is changed to a non-zero
value and passes the checking of blk_mq_request_started and then it will be handled by nvme_cancel_request.

Thanks
Jianchao