Re: [PATCH v2 1/3] mm/shuffle: don't move pages between zones and don't read garbage memmaps

[Wei Yang]

On Tue, Jun 23, 2020 at 09:55:43AM +0200, David Hildenbrand wrote:
>On 23.06.20 09:39, David Hildenbrand wrote:
>>> Hmm.. I thought this is the behavior for early section, while it looks current
>>> code doesn't work like this:
>>>
>>>        if (section_is_early && memmap)
>>>                free_map_bootmem(memmap);
>>>        else
>>> 	       depopulate_section_memmap(pfn, nr_pages, altmap);
>>>
>>> section_is_early is always "true" for early section, while memmap is not-NULL
>>> only when sub-section map is empty.
>>>
>>> If my understanding is correct, when we remove a sub-section in early section,
>>> the code would call depopulate_section_memmap(), which in turn free related
>>> memmap. By removing the memmap, the return value from pfn_to_online_page() is
>>> not a valid one.
>> 
>> I think you're right, and pfn_valid() would also return true, as it is
>> an early section. This looks broken.
>> 
>>>
>>> Maybe we want to write the code like this:
>>>
>>>        if (section_is_early)
>>>                if (memmap)
>>>                        free_map_bootmem(memmap);
>>>        else
>>> 	       depopulate_section_memmap(pfn, nr_pages, altmap);
>>>
>> 
>> I guess that should be the way to go
>> 
>> @Dan, I think what Wei proposes here is correct, right? Or how does it
>> work in the VMEMMAP case with early sections?
>> 
>
>Especially, if you would re-hot-add, section_activate() would assume
>there is a memmap, it must not be removed.
>

You are right here. I didn't notice it.

>@Wei, can you send a patch?
>

Sure, let me prepare for it.

>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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