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

[Qi Zheng]

On 2023/10/23 11:10, Huang, Ying wrote:
> Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> writes:
>
> > Hi Ying,
> >
> > On 2023/10/23 09:18, Huang, Ying wrote:
> > > 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.
> >
> > 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@xxxxxxxxxxxxx/
> > [2]. https://lore.kernel.org/all/20231017062215.171670-1-rppt@xxxxxxxxxx/
> >
> > ```

Hi Andrew, can you help modify the commit message to this? :)

Thanks,
Qi

> This is helpful.  Thanks!
>
> --
> Best Regards,
> Huang, Ying
>
> > 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@xxxxxxxxxxxxx>
> > > > > > Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> > > > > [snip]
> > > > > --
> > > > > Best Regards,
> > > > > Huang, Ying