Re: [PATCH] mm: fix panic in __alloc_pages

[David Hildenbrand]

On 02.11.21 14:25, Michal Hocko wrote:
> On Tue 02-11-21 13:39:06, David Hildenbrand wrote:
>>>> Yes, but a zonelist cannot be correct for an offline node, where we might
>>>> not even have an allocated pgdat yet. No pgdat, no zonelist. So as soon as
>>>> we allocate the pgdat and set the node online (->hotadd_new_pgdat()), the zone lists have to be correct. And I can spot an build_all_zonelists() in hotadd_new_pgdat().
>>>
>>> Yes, that is what I had in mind. We are talking about two things here.
>>> Memoryless nodes and offline nodes. The later sounds like a bug to me.
>>
>> Agreed. memoryless nodes should just have proper zonelists -- which
>> seems to be the case.
>>
>>>> Maybe __alloc_pages_bulk() and alloc_pages_node() should bail out directly
>>>> (VM_BUG()) in case we're providing an offline node with eventually no/stale pgdat as
>>>> preferred nid.
>>>
>>> Historically, those allocation interfaces were not trying to be robust
>>> against wrong inputs because that adds cpu cycles for everybody for
>>> "what if buggy" code. This has worked (surprisingly) well. Memory less
>>> nodes have brought in some confusion but this is still something that we
>>> can address on a higher level. Nobody give arbitrary nodes as an input.
>>> cpu_to_node might be tricky because it can point to a memory less node
>>> which along with __GFP_THISNODE is very likely not something anybody
>>> wants. Hence cpu_to_mem should be used for allocations. I hate we have
>>> two very similar APIs...
>>
>> To be precise, I'm wondering if we should do:
>>
>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
>> index 55b2ec1f965a..8c49b88336ee 100644
>> --- a/include/linux/gfp.h
>> +++ b/include/linux/gfp.h
>> @@ -565,7 +565,7 @@ static inline struct page *
>>  __alloc_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
>>  {
>>         VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
>> -       VM_WARN_ON((gfp_mask & __GFP_THISNODE) && !node_online(nid));
>> +       VM_WARN_ON(!node_online(nid));
>>
>>         return __alloc_pages(gfp_mask, order, nid, NULL);
>>  }
>>
>> (Or maybe VM_BUG_ON)
>>
>> Because it cannot possibly work and we'll dereference NULL later.
> 
> VM_BUG_ON would be silent for most configurations and crash would happen
> even without it so I am not sure about the additional value. VM_WARN_ON
> doesn't really add much on top - except it would crash in some
> configurations. If we really care to catch this case then we would have
> to do a reasonable fallback with a printk note and a dumps stack.

As I learned, VM_BUG_ON and friends are active for e.g., Fedora, which
can catch quite some issues early, before they end up in enterprise
distro kernels. I think it has value.

> Something like
> 	if (unlikely(!node_online(nid))) {
> 		pr_err("%d is an offline numa node and using it is a bug in a caller. Please report...\n");
> 		dump_stack();
> 		nid = numa_mem_id();
> 	}
> 
> But again this is adding quite some cycles to a hotpath of the page
> allocator. Is this worth it?

Don't think a fallback makes sense.

> 
>>> But something seems wrong in this case. cpu_to_node shouldn't return
>>> offline nodes. That is just a land mine. It is not clear to me how the
>>> cpu has been brought up so that the numa node allocation was left
>>> behind. As pointed in other email add_cpu resp. cpu_up is not it.
>>> Is it possible that the cpu bring up was only half way?
>>
>> I tried to follow the code (what sets a CPU present, what sets a CPU
>> online, when do we update cpu_to_node() mapping) and IMHO it's all a big
>> mess. Maybe it's clearer to people familiar with that code, but CPU
>> hotplug in general seems to be a confusing piece of (arch-specific) code.
> 
> Yes there are different arch specific parts that make this quite hard to
> follow.
> 
> I think we want to learn how exactly Alexey brought that cpu up. Because
> his initial thought on add_cpu resp cpu_up doesn't seem to be correct.
> Or I am just not following the code properly. Once we know all those
> details we can get in touch with cpu hotplug maintainers and see what
> can we do.

Yes.

> 
> Btw. do you plan to send a patch for pcp allocator to use cpu_to_mem?

You mean s/cpu_to_node/cpu_to_mem/ or also handling offline nids?

cpu_to_mem() corresponds to cpu_to_node() unless on ia64+ppc IIUC, so it
won't help for this very report.

> One last thing, there were some mentions of __GFP_THISNODE but I fail to
> see connection with the pcp allocator...

Me to. If pcpu would be using __GFP_THISNODE, we'd be hitting the
VM_WARN_ON but still crash.

-- 
Thanks,

David / dhildenb