linux-kernel - Re: [PATCH] scsi: core: Cap initial sdev queue depth at shost.can

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Date:   Thu, 22 Apr 2021 09:38:28 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     John Garry <john.garry@...wei.com>
Cc:     jejb@...ux.ibm.com, martin.petersen@...cle.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        kashyap.desai@...adcom.com, dgilbert@...erlog.com
Subject: Re: [PATCH] scsi: core: Cap initial sdev queue depth at
 shost.can_queue

On Tue, Apr 20, 2021 at 09:14:12AM +0100, John Garry wrote:
> On 20/04/2021 01:02, Ming Lei wrote:
> > On Tue, Apr 20, 2021 at 12:06:24AM +0800, John Garry wrote:
> > > Function sdev_store_queue_depth() enforces that the sdev queue depth cannot
> > > exceed shost.can_queue.
> > > 
> > > However, the LLDD may still set cmd_per_lun > can_queue, which leads to an
> > > initial sdev queue depth greater than can_queue.
> > > 
> > > Stop this happened by capping initial sdev queue depth at can_queue.
> > > 
> > > Signed-off-by: John Garry <john.garry@...wei.com>
> > > ---
> > > Topic originally discussed at:
> > > https://lore.kernel.org/linux-scsi/85dec8eb-8eab-c7d6-b0fb-5622747c5499@interlog.com/T/#m5663d0cac657d843b93d0c9a2374f98fc04384b9
> > > 
> > > Last idea there was to error/warn in scsi_add_host() for cmd_per_lun >
> > 
> 
> Hi Ming,
> 
> > No, that isn't my suggestion.
> 
> Right, it was what I mentioned.
> 
> > 
> > > can_queue. However, such a shost driver could still configure the sdev
> > > queue depth to be sound value at .slave_configure callback, so now thinking
> > > the orig patch better.
> > 
> > As I mentioned last time, why can't we fix ->cmd_per_lun in
> > scsi_add_host() using .can_queue?
> > 
> 
> I would rather not change the values which are provided from the driver. I
> would rather take the original values and try to use them in a sane way.
> 
> I have not seen other places where driver shost config values are modified
> by the core code.

Wrt. .cmd_per_lun, I think it is safe to modify it into one correct
depth because almost all drivers are just producer of .cmd_per_lun. And
except for debug purpose, there are only three consumers of .cmd_per_lun
in scsi, and all are for scsi_change_queue_depth():

	process_message()
	scsi_alloc_sdev()
	virtscsi_change_queue_depth()


Thanks,
Ming