scsi: storvsc: Don't assume cpu_possible_mask is dense

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@outlook.com>
Link: https://lore.kernel.org/r/20241003035333.49261-5-mhklinux@outlook.com
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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 eab32abf1f4921b2dbbf5ddf05e813682dbe33e8..5e3727e432d589ea2833141ee6881e79aff5cc6b 100644 (file)
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c
@@ -915,14 +915,13 @@ 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;