linux-kernel - Re: [PATCH v2 1/4] blk-flush: flush_rq should inherit first_rq's cmd

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Date:   Mon, 31 Jul 2023 22:02:39 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Christoph Hellwig <hch@....de>, hare@...e.de
Cc:     axboe@...nel.dk, ming.lei@...hat.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, zhouchengming@...edance.com
Subject: Re: [PATCH v2 1/4] blk-flush: flush_rq should inherit first_rq's
 cmd_flags

On 2023/7/31 14:09, Christoph Hellwig wrote:
> On Tue, Jul 25, 2023 at 09:00:59PM +0800, chengming.zhou@...ux.dev wrote:
>> From: Chengming Zhou <zhouchengming@...edance.com>
>>
>> The cmd_flags in blk_kick_flush() should inherit the original request's
>> cmd_flags, but the current code looks buggy to me:
> 
> Should it?  I know the code is kinda trying to do it, but does it really
> make sense?  Adding Hannes who originally added this inheritance and
> discussing the details below:

I'm not sure, actually I don't get what the current code is doing...
Hope Hannes could provide some details.

blk_flush_complete_seq(rq) -> blk_kick_flush(rq->cmd_flags)

flush_rq will use the cmd_flags of request which just complete a sequence,
there are three cases:

1. blk_insert_flush(rq):	rq is pending, wait for flush
2. flush_end_io(flush_rq):	rq flush seq done
3. mq_flush_data_end_io(rq):	rq data seq done

Only in the 1st case, the rq is the pending request that wait for flush_rq.
In the 2nd and 3rd cases, the rq has nothing to do with the next flush_rq?

So it's more reasonable for flush_rq to use its pending first_rq's cmd_flags?

> 
>>  	flush_rq->cmd_flags = REQ_OP_FLUSH | REQ_PREFLUSH;
>> -	flush_rq->cmd_flags |= (flags & REQ_DRV) | (flags & REQ_FAILFAST_MASK);
>> +	flush_rq->cmd_flags |= (first_rq->cmd_flags & REQ_DRV) |
>> +			       (first_rq->cmd_flags & REQ_FAILFAST_MASK);
> 
> Two cases here:
> 
>  1) REQ_FAILFAST_MASK:  I don't think this is actually set on flush request
>     currently, and even if it was applying it to the flush that serves more
>     than a single originating command seems wrong to me.
>  2) REQ_DRV is only set by drivers that have seen a bio.  For dm this
>     is used as REQ_DM_POLL_LIST which should never be set for a flush/fua
>     request.  For nvme-mpath it is REQ_NVME_MPATH, which is set in the
>     bio based driver and used for decision making in the I/O completion
>     handler.  So I guess this one actually does need to get passed
>     through.
> 

The commit 84fca1b0c461 ("block: pass failfast and driver-specific flags to
flush requests") says:
If flush requests are being sent to the device we need to inherit the
failfast and driver-specific flags, too, otherwise I/O will fail.

1) REQ_FAILFAST_MASK: agree, shouldn't set to the flush_rq I think?
2) REQ_DRV: I don't get why this flag not set would cause I/O fail?

Thanks!