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

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Message-ID: <d97ba697-16d9-4e62-86bd-31207b621b11@bytedance.com>
Date:   Mon, 23 Oct 2023 11:17:03 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     akpm@...ux-foundation.org, "Huang, Ying" <ying.huang@...el.com>
Cc:     rppt@...nel.org, david@...hat.com, vbabka@...e.cz, mhocko@...e.com,
        willy@...radead.org, mgorman@...hsingularity.net, mingo@...nel.org,
        aneesh.kumar@...ux.ibm.com, hannes@...xchg.org, osalvador@...e.de,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 2/2] mm: memory_hotplug: drop memoryless node from
 fallback lists



On 2023/10/23 11:10, Huang, Ying wrote:
> Qi Zheng <zhengqi.arch@...edance.com> writes:
> 
>> Hi Ying,
>>
>> On 2023/10/23 09:18, Huang, Ying wrote:
>>> Qi Zheng <zhengqi.arch@...edance.com> writes:
>>>
>>>> Hi Ying,
>>>>
>>>> On 2023/10/20 15:05, Huang, Ying wrote:
>>>>> Qi Zheng <zhengqi.arch@...edance.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/
>>
>> ```

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@...edance.com>
>>>>>> Acked-by: David Hildenbrand <david@...hat.com>
>>>>> [snip]
>>>>> --
>>>>> Best Regards,
>>>>> Huang, Ying