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

[Qi Zheng]

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/

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