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Date: Mon, 23 Sep 2019 17:15:19 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Yunsheng Lin <linyunsheng@huawei.com>
Subject: Re: [PATCH v6] numa: make node_to_cpumask_map() NUMA_NO_NODE aware
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On Tue, Sep 17, 2019 at 08:48:54PM +0800, Yunsheng Lin wrote:
> When passing the return value of dev_to_node() to cpumask_of_node()
> without checking if the device's node id is NUMA_NO_NODE, there is
> global-out-of-bounds detected by KASAN.
> 
> From the discussion [1], NUMA_NO_NODE really means no node affinity,
> which also means all cpus should be usable. So the cpumask_of_node()
> should always return all cpus online when user passes the node id as
> NUMA_NO_NODE, just like similar semantic that page allocator handles
> NUMA_NO_NODE.
> 
> But we cannot really copy the page allocator logic. Simply because the
> page allocator doesn't enforce the near node affinity. It just picks it
> up as a preferred node but then it is free to fallback to any other numa
> node. This is not the case here and node_to_cpumask_map will only restrict
> to the particular node's cpus which would have really non deterministic
> behavior depending on where the code is executed. So in fact we really
> want to return cpu_online_mask for NUMA_NO_NODE.
> 
> Also there is a debugging version of node_to_cpumask_map() for x86 and
> arm64, which is only used when CONFIG_DEBUG_PER_CPU_MAPS is defined, this
> patch changes it to handle NUMA_NO_NODE as normal node_to_cpumask_map().
> 
> [1] https://lore.kernel.org/patchwork/patch/1125789/

That is bloody unusable, don't do that. Use:

  https://lkml.kernel.org/r/$MSGID

if anything. Then I can find it in my local mbox without having to
resort to touching a mouse and shitty browser software.

(also patchwork is absolute crap for reading email threads)

Anyway, I found it -- I think, I refused to click the link. I replied
there.

> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Acked-by: Michal Hocko <mhocko@suse.com>



> diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
> index 4123100e..9859acb 100644
> --- a/arch/x86/mm/numa.c
> +++ b/arch/x86/mm/numa.c
> @@ -861,6 +861,9 @@ void numa_remove_cpu(int cpu)
>   */
>  const struct cpumask *cpumask_of_node(int node)
>  {
> +	if (node == NUMA_NO_NODE)
> +		return cpu_online_mask;

This mandates the caller holds cpus_read_lock() or something, I'm pretty
sure that if I put:

	lockdep_assert_cpus_held();

here, it comes apart real quick. Without holding the cpu hotplug lock,
the online mask is gibberish.

> +
>  	if ((unsigned)node >= nr_node_ids) {
>  		printk(KERN_WARNING
>  			"cpumask_of_node(%d): (unsigned)node >= nr_node_ids(%u)\n",

I still think this makes absolutely no sense what so ever.