Re: [PATCH v3 10/11] scsi: scsi_debug: Only allow sdebug_max_queue be modified when no shosts

[Douglas Gilbert]

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@xxxxxxxxxx>
Acked-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>

Thanks.