linux-kernel - Re: [PATCH v3 10/11] scsi: scsi_debug: Only allow sdebug_max

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Message-ID: <f2ece258-c14f-69d2-f302-956bff2c921d@interlog.com>
Date:   Mon, 3 Apr 2023 01:23:55 -0400
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     John Garry <john.g.garry@...cle.com>, jejb@...ux.ibm.com,
        martin.petersen@...cle.com
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        bvanassche@....org
Subject: Re: [PATCH v3 10/11] scsi: scsi_debug: Only allow sdebug_max_queue be
 modified when no shosts

On 2023-03-27 03:43, John Garry wrote:
> The shost->can_queue value is initially used to set per-HW queue context
> tag depth in the block layer. This ensures that the shost is not sent too
> many commands which it can deal with. However lowering sdebug_max_queue
> separately means that we can easily overload the shost, as in the following
> example:
> 
> $ cat /sys/bus/pseudo/drivers/scsi_debug/max_queue
> 192
> $ cat /sys/class/scsi_host/host0/can_queue
> 192
> $ echo 100 > /sys/bus/pseudo/drivers/scsi_debug/max_queue
> $ cat /sys/class/scsi_host/host0/can_queue
> 192
> $ fio --filename=/dev/sda --direct=1 --rw=read --bs=4k --iodepth=256
> --runtime=1200 --numjobs=10 --time_based --group_reporting
> --name=iops-test-job --eta-newline=1 --readonly    --ioengine=io_uring
> --hipri --exitall_on_error
> iops-test-job: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=io_uring, iodepth=256
> ...
> fio-3.28
> Starting 10 processes
> [  111.269885] scsi_io_completion_action: 400 callbacks suppressed
> [  111.269885] blk_print_req_error: 400 callbacks suppressed
> [  111.269889] I/O error, dev sda, sector 440 op 0x0:(READ) flags 0x1200000 phys_seg 1 prio class 2
> [  111.269892] sd 0:0:0:0: [sda] tag#132 FAILED Result: hostbyte=DID_ABORT driverbyte=DRIVER_OK cmd_age=0s
> [  111.269897] sd 0:0:0:0: [sda] tag#132 CDB: Read(10) 28 00 00 00 01 68 00 00 08 00
> [  111.277058] I/O error, dev sda, sector 360 op 0x0:(READ) flags 0x1200000 phys_seg 1 prio class 2
> 
> [...]
> 
> Ensure that this cannot happen by allowing sdebug_max_queue be modified
> only when we have no shosts. As such, any shost->can_queue value will
> match sdebug_max_queue, and sdebug_max_queue cannot be modified separately.
> 
> Since retired_max_queue is no longer set, remove support.
> 
> Continue to apply the restriction that sdebug_host_max_queue cannot be
> modified when sdebug_host_max_queue is set. Adding support for that would
> mean extra code, and no one has complained about this restriction
> previously.
> 
> A command like the following may be used to remove a shost:
> echo -1 > /sys/bus/pseudo/drivers/scsi_debug/add_host
> 
> Signed-off-by: John Garry <john.g.garry@...cle.com>
Acked-by: Douglas Gilbert <dgilbert@...erlog.com>

Thanks.