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Subject[PATCH 1/1] scsi: storvsc: Fix calculation of sub-channel count
Date
When the number of sub-channels offered by Hyper-V is >= the number
of CPUs in the VM, calculate the correct number of sub-channels.
The current code produces one too many.

This scenario arises only when the number of CPUs is artificially
restricted (for example, with maxcpus=<n> on the kernel boot line),
because Hyper-V normally offers a sub-channel count < number of CPUs.
While the current code doesn't break, the extra sub-channel is
unbalanced across the CPUs (for example, a total of 5 channels on
a VM with 4 CPUs).

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
---
drivers/scsi/storvsc_drv.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 84380ba..fbaa11a 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c
@@ -674,7 +674,17 @@ static void handle_multichannel_storage(struct hv_device *device, int max_chns)
struct vstor_packet *vstor_packet;
int ret, t;

- num_sc = ((max_chns > num_cpus) ? num_cpus : max_chns);
+ /*
+ * If the number of CPUs is artificially restricted, such as
+ * with maxcpus=1 on the kernel boot line, Hyper-V could offer
+ * sub-channels >= the number of CPUs. These sub-channels
+ * should not be created. The primary channel is already created
+ * and assigned to one CPU, so check against # CPUs - 1.
+ */
+ num_sc = min((num_cpus - 1), max_chns);
+ if (!num_sc)
+ return;
+
stor_device = get_out_stor_device(device);
if (!stor_device)
return;
--
1.8.3.1
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