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

[Qi Zheng]

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/

```

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