Re: [PATCH v3 2/2] mm: memory_hotplug: drop memoryless node from fallback lists

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi Ying,

On 2023/10/23 09:18, Huang, Ying wrote:
> Qi Zheng <zhengqi.arch@bytedance.com> writes:
> 
>> Hi Ying,
>>
>> On 2023/10/20 15:05, Huang, Ying wrote:
>>> Qi Zheng <zhengqi.arch@bytedance.com> writes:
>>>
>>>> In offline_pages(), if a node becomes memoryless, we
>>>> will clear its N_MEMORY state by calling node_states_clear_node().
>>>> But we do this after rebuilding the zonelists by calling
>>>> build_all_zonelists(), which will cause this memoryless node to
>>>> still be in the fallback list of other nodes.
>>> For fallback list, do you mean pgdat->node_zonelists[]?  If so, in
>>> build_all_zonelists
>>>     __build_all_zonelists
>>>       build_zonelists
>>>         build_zonelists_in_node_order
>>>           build_zonerefs_node
>>> populated_zone() will be checked before adding zone into zonelist.
>>> So, IIUC, we will not try to allocate from the memory less node.
>>
>> Normally yes, but if it is the weird topology mentioned in [1], it's
>> possible to allocate memory from it, it is a memoryless node, but it
>> also has memory.
>>
>> In addition to the above case, I think it's reasonable to remove
>> memory less node from node_order[] in advance. In this way it will
>> not to be traversed in build_zonelists_in_node_order().
>>
>> [1]. https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
> 
> Got it!  Thank you for information.  I think that it may be good to
> include this in the patch description to avoid potential confusing in
> the future.

OK, maybe the commit message can be changed to the following:

```
In offline_pages(), if a node becomes memoryless, we
will clear its N_MEMORY state by calling node_states_clear_node().
But we do this after rebuilding the zonelists by calling
build_all_zonelists(), which will cause this memoryless node to
still be in the fallback nodes (node_order[]) of other nodes.

To drop memoryless nodes from fallback nodes in this case, just
call node_states_clear_node() before calling build_all_zonelists().

In this way, we will not try to allocate pages from memoryless
node0, then the panic mentioned in [1] will also be fixed. Even though
this problem has been solved by dropping the NODE_MIN_SIZE constrain
in x86 [2], it would be better to fix it in the core MM as well.

[1]. 
https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
[2]. https://lore.kernel.org/all/20231017062215.171670-1-rppt@kernel.org/

```

Thanks,
Qi

> 
> --
> Best Regards,
> Huang, Ying
> 
>> Thanks,
>> Qi
>>
>>
>>> --
>>> Best Regards,
>>> Huang, Ying
>>>
>>>> This will incur
>>>> some runtime overhead.
>>>>
>>>> To drop memoryless node from fallback lists in this case, just
>>>> call node_states_clear_node() before calling build_all_zonelists().
>>>>
>>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>>> Acked-by: David Hildenbrand <david@redhat.com>
>>> [snip]
>>> --
>>> Best Regards,
>>> Huang, Ying