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SubjectRe: [PATCH] mm/spase: never partially remove memmap for early section
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@linux.alibaba.com>
>
>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.

>
>> ---
>> mm/sparse.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index b2b9a3e34696..1a0069f492f5 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -825,10 +825,10 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>> ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
>> }
>>
>> - if (section_is_early && memmap)
>> - free_map_bootmem(memmap);
>> - else
>> + if (!section_is_early)
>
>This begs a comment.
>
>> depopulate_section_memmap(pfn, nr_pages, altmap);
>> + else if (memmap)
>> + free_map_bootmem(memmap);
>>
>> if (empty)
>> ms->section_mem_map = (unsigned long)NULL;
>> --
>> 2.20.1 (Apple Git-117)
>>
>
>--
>Michal Hocko
>SUSE Labs

--
Wei Yang
Help you, Help me

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