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

[Michael Kelley]

From: John Garry <john.garry@xxxxxxxxxx> Sent: Wednesday, October 6, 2021 1:17 AM
> 
> 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.
> 

The calculated value of can_queue is not realistic.  The blk-mq layer
caps the number of tags at 10240, so the excessively large value
calculated here didn't definitively break anything, though it can be
poor from a performance tuning standpoint. The algorithm used here
is fairly broken, particularly in VMs with large CPU counts.  I have an
effort underway to fix it, but its part of a bigger set of changes to also
do a better job on the perf tuning aspects.

> >
> > 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).

I can see that making can_queue be 16 bits would make sense.
And it also seems that both cmd_per_lun and can_queue should be
unsigned, though I don't the implications of making such a change.

But in today's world where cmd_per_lun is "short" and can_queue
is "int",  ea2f0f77538c seems incorrect to me.  The comparison should
be done as "int", not "short", in order to prevent the truncation
problem with can_queue that Dexuan's patch is trying to address.
The result will always fit in back into the "short" cmd_per_lun since
it is calculating a "min" function.

> 
> Thanks,
> John
> 
> 
> > +	if (scsi_driver.can_queue > SHRT_MAX)
> > +		scsi_driver.can_queue = SHRT_MAX;
> > +

This fix works, but is a more of a temporary hack until I can finish
a larger overhaul of the algorithm.   But for now, I think the better
fix is for ea2f0f77538c to do the comparison as "int" instead of "short".

Michael

> >   	host = scsi_host_alloc(&scsi_driver,
> >   			       sizeof(struct hv_host_device));
> >   	if (!host)
> >