Re: [PATCH] mm/spase: never partially remove memmap for early section

[Wei Yang]

On Wed, Jun 24, 2020 at 09:10:09AM -0700, Dan Williams wrote:
>On Tue, Jun 23, 2020 at 11:14 PM Wei Yang
><richard.weiyang@xxxxxxxxxxxxxxxxx> wrote:
>>
>> On Tue, Jun 23, 2020 at 05:18:28PM +0200, Michal Hocko wrote:
>> >On Tue 23-06-20 17:42:58, Wei Yang wrote:
>> >> For early sections, we assumes its memmap will never be partially
>> >> removed. But current behavior breaks this.
>> >>
>> >> Let's correct it.
>> >>
>> >> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>> >> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
>> >
>> >Can a user trigger this or is this a theoretical bug?
>>
>> Let me rewrite the changelog a little. Look forward any comments.
>>
>>    For early sections, its memmap is handled specially even sub-section is
>>    enabled. The memmap could only be populated as a whole.
>>
>>    Quoted from the comment of section_activate():
>>
>>        * The early init code does not consider partially populated
>>        * initial sections, it simply assumes that memory will never be
>>        * referenced.  If we hot-add memory into such a section then we
>>        * do not need to populate the memmap and can simply reuse what
>>        * is already there.
>>
>>    While current section_deactivate() breaks this rule. When hot-remove a
>>    sub-section, section_deactivate() would depopulate its memmap. The
>>    consequence is if we hot-add this subsection again, its memmap never get
>>    proper populated.
>
>Ok, forgive the latency as re-fetched this logic into my mental cache.
>So what I was remembering was the initial state of the code that
>special cased early sections, and that still seems to be the case in
>pfn_valid(). IIRC early_sections / bootmem are blocked from being
>removed entirely. Partial / subsection removals are ok.

Would you mind giving more words? Partial subsection removal is ok, so no need
to fix this?

-- 
Wei Yang
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