linux-kernel - [PATCH AUTOSEL 4.14 23/35] scsi: storvsc: Fix calculation of sub-channel count

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Date:   Wed, 24 Apr 2019 10:46:57 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Michael Kelley <mikelley@...rosoft.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Sasha Levin <sashal@...nel.org>, linux-hyperv@...r.kernel.org,
        linux-scsi@...r.kernel.org
Subject: [PATCH AUTOSEL 4.14 23/35] scsi: storvsc: Fix calculation of sub-channel count

From: Michael Kelley <mikelley@...rosoft.com>

[ Upstream commit 382e06d11e075a40b4094b6ef809f8d4bcc7ab2a ]

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@...rosoft.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
Reviewed-by: Long Li <longli@...rosoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@...cle.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/scsi/storvsc_drv.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index beb585ddc07d..5adeb1e4b186 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c
@@ -658,13 +658,22 @@ static void handle_sc_creation(struct vmbus_channel *new_sc)
 static void  handle_multichannel_storage(struct hv_device *device, int max_chns)
 {
 	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;
-- 
2.19.1