Subject: Re: [PATCH v2 2/9] x86: numa: check the node id consistently for x86

From: Peter Zijlstra <[email-masked]>
Date: Mon, 2 Sep 2019 09:25:42 +0200

On Mon, Sep 02, 2019 at 01:46:51PM +0800, Yunsheng Lin wrote:
> On 2019/9/1 0:12, Peter Zijlstra wrote:

> > 1) because even it is not set, the device really does belong to a node.
> > It is impossible a device will have magic uniform access to memory when
> > CPUs cannot.
> 
> So it means dev_to_node() will return either NUMA_NO_NODE or a
> valid node id?

NUMA_NO_NODE := -1, which is not a valid node number. It is also, like I
said, not a valid device location on a NUMA system.

Just because ACPI/BIOS is shit, doesn't mean the device doesn't have a
node association. It just means we don't know and might have to guess.

> > 2) is already true today, cpumask_of_node() requires a valid node_id.
> 
> Ok, most of the user does check node_id before calling
> cpumask_of_node(), but does a little different type of checking:
> 
> 1) some does " < 0" check;
> 2) some does "== NUMA_NO_NODE" check;
> 3) some does ">= MAX_NUMNODES" check;
> 4) some does "< 0 || >= MAX_NUMNODES || !node_online(node)" check.

The one true way is:

	'(unsigned)node_id >= nr_node_ids'

> > 3) is just wrong and increases overhead for everyone.
> 
> Ok, cpumask_of_node() is also used in some critical path such
> as scheduling, which may not need those checking, the overhead
> is unnecessary.
> 
> But for non-critical path such as setup or configuration path,
> it better to have consistent checking, and also simplify the
> user code that calls cpumask_of_node().
> 
> Do you think it is worth the trouble to add a new function
> such as cpumask_of_node_check(maybe some other name) to do
> consistent checking?
> 
> Or caller just simply check if dev_to_node()'s return value is
> NUMA_NO_NODE before calling cpumask_of_node()?

It is not a matter of convenience. The function is called
cpumask_of_node(), when node < 0 || node >= nr_node_ids, it is not a
valid node, therefore the function shouldn't return anything except an
error.

Also note that the CONFIG_DEBUG_PER_CPU_MAPS version of
cpumask_of_node() already does this (although it wants the below fix).

---
diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
index e6dad600614c..5f49c10201c7 100644
--- a/arch/x86/mm/numa.c
+++ b/arch/x86/mm/numa.c
@@ -861,7 +861,7 @@ void numa_remove_cpu(int cpu)
  */
 const struct cpumask *cpumask_of_node(int node)
 {
-	if (node >= nr_node_ids) {
+	if ((unsigned)node >= nr_node_ids) {
 		printk(KERN_WARNING
 			"cpumask_of_node(%d): node > nr_node_ids(%u)\n",
 			node, nr_node_ids);

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