On 06/10/2021 16:01, Michael Kelley wrote:

From: John Garry <redacted> Sent: Wednesday, October 6, 2021 1:17 AM

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On 06/10/2021 08:03, Dexuan Cui wrote:

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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, 

nit: 1024, I think

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.

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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@vger.kernel.org
Signed-off-by: Dexuan Cui <decui@microsoft.com>
---
   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.

Yeah, right. I think can_queue values > short_max was considered outside 
the realms of what is realistic then, hence my sloppy programming.

The result will always fit in back into the "short" cmd_per_lun since
it is calculating a "min" function.

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Thanks,
John

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+	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".

That seems better to me. But Let's wait for other possible opinion.

Thanks,
John