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Subject: Re: [PATCH v2 1/3] mm/shuffle: don't move pages between zones and
 don't read garbage memmaps
From:   David Hildenbrand <david@redhat.com>
To:     Wei Yang <richard.weiyang@gmail.com>
Cc:     Wei Yang <richard.weiyang@linux.alibaba.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Michal Hocko <mhocko@suse.com>, stable@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Minchan Kim <minchan@kernel.org>,
        Huang Ying <ying.huang@intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Dan Williams <dan.j.williams@intel.com>
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Message-ID: <c0b62330-11d3-e628-a811-b54789d8f182@redhat.com>
Date:   Tue, 23 Jun 2020 09:55:43 +0200
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Sender: linux-kernel-owner@vger.kernel.org
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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.

@Wei, can you send a patch?

-- 
Thanks,

David / dhildenb