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

[Huang, Ying]

Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> writes:

> Hi Ying,
>
> On 2023/10/20 15:05, Huang, Ying wrote:
>> Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx/

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.

--
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@xxxxxxxxxxxxx>
>>> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>> [snip]
>> --
>> Best Regards,
>> Huang, Ying