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>

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 84380ba..e186743 100644 (file)
--- 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;