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

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Message-ID: <22bedb25-63ee-3106-f37d-b4ef19ac9e6f@suse.de>
Date:   Mon, 20 Jun 2022 13:56:34 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Christoph Hellwig <hch@....de>
Cc:     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 13:24, Damien Le Moal wrote:
> 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.
> 

Tsk. I was hoping to be able to remove it (especially 
scsi_eh_get_sense()), but looks as if we actually do need it.
But it might be not a bad idea to have scsi_eh_get_sense() to run 
independent on the SCSI EH stuff; returning with a sense code is not 
necessary an error, so there are reasons for not always invoking SCSI EH.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@...e.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Felix Imendörffer