linux-kernel - Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command handling

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Date:   Mon, 20 Jun 2022 20:24:48 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Hannes Reinecke <hare@...e.de>, John Garry <john.garry@...wei.com>,
        Bart Van Assche <bvanassche@....org>, axboe@...nel.dk,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, brking@...ibm.com,
        linux-block@...r.kernel.org, linux-ide@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        chenxiang66@...ilicon.com
Subject: Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command
 handling

On 6/20/22 18:05, Christoph Hellwig wrote:
> On Mon, Jun 20, 2022 at 06:02:30PM +0900, Damien Le Moal wrote:
>> So reserving a tag/req to be able to do NCQ at the cost of max qd being 31
>> works for that. We could keep max qd at 32 by creating one more "fake" tag
>> and having a request for it, that is, having the fake tag visible to the
>> block layer as a reserved tag, as John's series is doing, but for the
>> reserved tags, we actually need to use an effective tag (qc->hw_tag) when
>> issuing the commands. And for that, we can reuse the tag of one of the
>> failed commands.
> 
> Take a look at the magic flush request in blk-flush.c, which is
> preallocated but borrows a tag from the request that wants a pre- or
> post-flush.  The logic is rather ugly, but maybe it might actually
> become cleaner by generalizing it a bit.

Thanks. Will check.
I am also looking at scsi_unjam_host() and scsi_eh_get_sense(). These
reuse a scsi command to do eh operations. So I could use that too, modulo
making it work outside of eh context to keep the command flow intact.

-- 
Damien Le Moal
Western Digital Research