linux-kernel - Re: [PATCH 01/11] blk-mq: Add blk_mq_init_queue

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Date:   Tue, 22 Mar 2022 15:17:24 +0000
From:   John Garry <john.garry@...wei.com>
To:     Hannes Reinecke <hare@...e.de>, Christoph Hellwig <hch@....de>
CC:     <axboe@...nel.dk>, <damien.lemoal@...nsource.wdc.com>,
        <bvanassche@....org>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <ming.lei@...hat.com>,
        <chenxiang66@...ilicon.com>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-ide@...r.kernel.org>,
        <linux-scsi@...r.kernel.org>, <dm-devel@...hat.com>,
        <beanhuo@...ron.com>
Subject: Re: [PATCH 01/11] blk-mq: Add blk_mq_init_queue_ops()

On 22/03/2022 14:03, Hannes Reinecke wrote:
>>
>> As mentioned in the cover letter response, it just seems best to keep 
>> the normal scsi_cmnd payload but have other means to add on the 
>> internal command data, like using host_scribble or scsi_cmnd priv data.
>>
> Well; I found that most drivers I had been looking at the scsi command 
> payload isn't used at all; the drivers primarily cared about the 
> (driver-provided) payload, and were completely ignoring the scsi command 
> payload.
> 
> Similar for ATA/libsas: you basically never issue real scsi commands, 
> but either 'raw' ATA requests or SCSI TMFs. None of which are scsi 
> commands, so providing them is a bit of a waste.
> 
> (And causes irritations, too, as a scsi command requires associated 
> pointers like ->device etc to be set up. Which makes it tricky to use 
> for the initial device setup.)

A problem I see is that in scsi_mq_init_request() we allocate memories 
like sense_buffer and prot_sdb and store the pointers in the scsi_cmnd 
payload. If we then reuse a scsi_cmnd payload as an "internal" command 
payload then this data may be lost.

It might be possible to reuse the scsi cmnd payload for the "internal", 
but I would rather not get hung up on it now if possible.

Thanks,
John