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

From: Yunsheng Lin <linyunsheng@huawei•com>
To: Peter Zijlstra <peterz@infradead•org>
Cc: dalias@libc•org, linux-sh@vger•kernel.org,
	catalin.marinas@arm•com, dave.hansen@linux•intel.com,
	heiko.carstens@de•ibm.com, linuxarm@huawei•com,
	jiaxun.yang@flygoat•com, linux-kernel@vger•kernel.org,
	mwb@linux•vnet.ibm.com, paulus@samba•org, hpa@zytor•com,
	sparclinux@vger•kernel.org, chenhc@lemote•com, will@kernel•org,
	linux-s390@vger•kernel.org, ysato@users•sourceforge.jp,
	mpe@ellerman•id.au, x86@kernel•org, rppt@linux•ibm.com,
	borntraeger@de•ibm.com, dledford@redhat•com, mingo@redhat•com,
	jeffrey.t.kirsher@intel•com, benh@kernel•crashing.org,
	jhogan@kernel•org, nfont@linux•vnet.ibm.com, mattst88@gmail•com,
	len.brown@intel•com, gor@linux•ibm.com,
	anshuman.khandual@arm•com, ink@jurassic•park.msu.ru, cai@lca•pw,
	luto@kernel•org, tglx@linutronix•de,
	naveen.n.rao@linux•vnet.ibm.com,
	linux-arm-kernel@lists•infradead.org, rth@twiddle•net,
	axboe@kernel•dk, robin.murphy@arm•com,
	linux-mips@vger•kernel.org, ralf@linux-mips•org,
	tbogendoerfer@suse•de, paul.burton@mips•com,
	linux-alpha@vger•kernel.org, bp@alien8•de,
	akpm@linux-foundation•org, linuxppc-dev@lists•ozlabs.org,
	davem@davemloft•net
Subject: Re: [PATCH v2 2/9] x86: numa: check the node id consistently for x86
Date: Mon, 2 Sep 2019 13:46:51 +0800	[thread overview]
Message-ID: <ae64285f-5134-4147-7b02-34bb5d519e8c@huawei.com> (raw)
In-Reply-To: <20190831161247.GM2369@hirez.programming.kicks-ass.net>

On 2019/9/1 0:12, Peter Zijlstra wrote:
> On Sat, Aug 31, 2019 at 06:09:39PM +0800, Yunsheng Lin wrote:
>>
>>
>> On 2019/8/31 16:55, Peter Zijlstra wrote:
>>> On Sat, Aug 31, 2019 at 01:58:16PM +0800, Yunsheng Lin wrote:
>>>> According to Section 6.2.14 from ACPI spec 6.3 [1], the setting
>>>> of proximity domain is optional, as below:
>>>>
>>>> This optional object is used to describe proximity domain
>>>> associations within a machine. _PXM evaluates to an integer
>>>> that identifies a device as belonging to a Proximity Domain
>>>> defined in the System Resource Affinity Table (SRAT).
>>>
>>> That's just words.. what does it actually mean?
>>
>> It means the dev_to_node(dev) may return -1 if the bios does not
>> implement the proximity domain feature, user may use that value
>> to call cpumask_of_node and cpumask_of_node does not protect itself
>> from node id being -1, which causes out of bound access.
> 
>>>> @@ -69,6 +69,12 @@ extern const struct cpumask *cpumask_of_node(int node);
>>>>  /* Returns a pointer to the cpumask of CPUs on Node 'node'. */
>>>>  static inline const struct cpumask *cpumask_of_node(int node)
>>>>  {
>>>> +	if (node >= nr_node_ids)
>>>> +		return cpu_none_mask;
>>>> +
>>>> +	if (node < 0 || !node_to_cpumask_map[node])
>>>> +		return cpu_online_mask;
>>>> +
>>>>  	return node_to_cpumask_map[node];
>>>>  }
>>>>  #endif
>>>
>>> I _reallly_ hate this. Users are expected to use valid numa ids. Now
>>> we're adding all this checking to all users. Why do we want to do that?
>>
>> As above, the dev_to_node(dev) may return -1.
>>
>>>
>>> Using '(unsigned)node >= nr_nods_ids' is an error.
>>
>> 'node >= nr_node_ids' can be dropped if all user is expected to not call
>> cpumask_of_node with node id greater or equal to nr_nods_ids.
> 
> you copied my typo :-)

I did note the typo, corrected the first one, but missed the second one :)

> 
>> From what I can see, the problem can be fixed in three place:
>> 1. Make user dev_to_node return a valid node id even when proximity
>>    domain is not set by bios(or node id set by buggy bios is not valid),
>>    which may need info from the numa system to make sure it will return
>>    a valid node.
>>
>> 2. User that call cpumask_of_node should ensure the node id is valid
>>    before calling cpumask_of_node, and user also need some info to
>>    make ensure node id is valid.
>>
>> 3. Make sure cpumask_of_node deal with invalid node id as this patchset.
>>
>> Which one do you prefer to make sure node id is valid, or do you
>> have any better idea?
>>
>> Any detail advice and suggestion will be very helpful, thanks.
> 
> 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?

> 
> 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.

> 
> 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()?

> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists•infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 
> .
> 

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists•infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel