Re: [PATCH 1/3] mm/sparsemem: adjust memmap only for SPARSEMEM_VMEMMAP

[Wei Yang]

On Thu, Feb 06, 2020 at 06:00:13PM -0800, Dan Williams wrote:
>On Thu, Feb 6, 2020 at 3:17 PM Wei Yang <richardw.yang@xxxxxxxxxxxxxxx> wrote:
>>
>> Only when SPARSEMEM_VMEMMAP is set, memmap returned from
>> section_activate() points to sub-section page struct. Otherwise, memmap
>> already points to the whole section page struct.
>>
>> This means only for SPARSEMEM_VMEMMAP, we need to adjust memmap for
>> sub-section case.
>>
>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>> CC: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>>  mm/sparse.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index 586d85662978..b5da121bdd6e 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -886,7 +886,8 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
>>         section_mark_present(ms);
>>
>>         /* Align memmap to section boundary in the subsection case */
>> -       if (section_nr_to_pfn(section_nr) != start_pfn)
>> +       if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
>> +               section_nr_to_pfn(section_nr) != start_pfn)
>
>Aren't we assured that start_pfn is always section aligned in the
>SPARSEMEM case? That's the role of check_pfn_span(). Does the change
>have a runtime impact or is this just theoretical?

You are right, I missed this point.

Thanks

-- 
Wei Yang
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