netdev - [PATCH 4/5] scsi: storvsc: Don't assume cpu_possible

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Message-Id: <20241003035333.49261-5-mhklinux@outlook.com>
Date: Wed,  2 Oct 2024 20:53:32 -0700
From: mhkelley58@...il.com
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Subject: [PATCH 4/5] scsi: storvsc: Don't assume cpu_possible_mask is dense

From: Michael Kelley <mhklinux@...look.com>

Current code allocates the stor_chns array with size num_possible_cpus().
This code assumes cpu_possible_mask is dense, which is not true in
the general case per [1]. If cpu_possible_mask is sparse, the array
might be indexed by a value beyond the size of the array.

However, the configurations that Hyper-V provides to guest VMs on x86
and ARM64 hardware, in combination with how architecture specific code
assigns Linux CPU numbers, *does* always produce a dense cpu_possible_mask.
So the dense assumption is not currently causing failures. But for
robustness against future changes in how cpu_possible_mask is populated,
update the code to no longer assume dense.

The correct approach is to allocate and initialize the array using size
"nr_cpu_ids". While this leaves unused array entries corresponding to
holes in cpu_possible_mask, the holes are assumed to be minimal and hence
the amount of memory wasted by unused entries is minimal.

[1] https://lore.kernel.org/lkml/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/

Signed-off-by: Michael Kelley <mhklinux@...look.com>
---
 drivers/scsi/storvsc_drv.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 11b3fc3b24c9..f2beb6b23284 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c
@@ -917,14 +917,13 @@ static int storvsc_channel_init(struct hv_device *device, bool is_fc)
 
 	/*
 	 * Allocate state to manage the sub-channels.
-	 * We allocate an array based on the numbers of possible CPUs
-	 * (Hyper-V does not support cpu online/offline).
-	 * This Array will be sparseley populated with unique
-	 * channels - primary + sub-channels.
-	 * We will however populate all the slots to evenly distribute
-	 * the load.
+	 * We allocate an array based on the number of CPU ids. This array
+	 * is initially sparsely populated for the CPUs assigned to channels:
+	 * primary + sub-channels. As I/Os are initiated by different CPUs,
+	 * the slots for all online CPUs are populated to evenly distribute
+	 * the load across all channels.
 	 */
-	stor_device->stor_chns = kcalloc(num_possible_cpus(), sizeof(void *),
+	stor_device->stor_chns = kcalloc(nr_cpu_ids, sizeof(void *),
 					 GFP_KERNEL);
 	if (stor_device->stor_chns == NULL)
 		return -ENOMEM;
-- 
2.25.1