scsi: storvsc: Fix calculation of sub-channel count

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>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

1 files changed, 11 insertions, 2 deletions

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 84380bae20f1..e186743033f4 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c
@@ -668,13 +668,22 @@ static void  handle_multichannel_storage(struct hv_device *device, int max_chns)
 {
         struct device *dev = &device->device;
         struct storvsc_device *stor_device;
-        int num_cpus = num_online_cpus();
         int num_sc;
         struct storvsc_cmd_request *request;
         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((int)(num_online_cpus() - 1), max_chns);
+        if (!num_sc)
+                return;
+
         stor_device = get_out_stor_device(device);
         if (!stor_device)
                 return;