Re: [PATCH] scsi: storvsc: Cap scsi_driver.can_queue to fix a hang issue during boot

[John Garry]

On 06/10/2021 08:03, Dexuan Cui wrote:
> After commit ea2f0f77538c, a 416-CPU VM running on Hyper-V hangs during
> boot because scsi_add_host_with_dma() sets shost->cmd_per_lun to a
> negative number:
> 	'max_outstanding_req_per_channel' is 352,
> 	'max_sub_channels' is (416 - 1) / 4 = 103, so in storvsc_probe(),
> scsi_driver.can_queue = 352 * (103 + 1) * (100 - 10) / 100 = 32947, which
> is bigger than SHRT_MAX (i.e. 32767).

Out of curiosity, are these values realistic? You're capping can_queue 
just because of a data size issue, so, if these values are realistic, 
seems a weak reason.

> Fix the hang issue by capping scsi_driver.can_queue.
>
> Add the below Fixed tag though ea2f0f77538c itself is good.
>
> Fixes: ea2f0f77538c ("scsi: core: Cap scsi_host cmd_per_lun at can_queue")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> ---
>   drivers/scsi/storvsc_drv.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index ebbbc1299c62..ba374908aec2 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -1976,6 +1976,16 @@ static int storvsc_probe(struct hv_device *device,
>   				(max_sub_channels + 1) *
>   				(100 - ring_avail_percent_lowater) / 100;
>   
> +	/*
> +	 * v5.14 (see commit ea2f0f77538c) implicitly requires that
> +	 * scsi_driver.can_queue should not exceed SHRT_MAX, otherwise
> +	 * scsi_add_host_with_dma() sets shost->cmd_per_lun to a negative
> +	 * number (note: the type of the "cmd_per_lun" field is "short"), and
> +	 * the system may hang during early boot.
> +	 */

The different data sizes for cmd_per_lun and can_queue are problematic here.

I'd be more inclined to set cmd_per_lun to the same data size as 
can_queue. We did discuss this when ea2f0f77538c was upstreamed 
(actually it was the other way around - setting can_queue to 16b).

Thanks,
John


> +	if (scsi_driver.can_queue > SHRT_MAX)
> +		scsi_driver.can_queue = SHRT_MAX;
> +
>   	host = scsi_host_alloc(&scsi_driver,
>   			       sizeof(struct hv_host_device));
>   	if (!host)