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

[Wei Yang <richard.weiyang@linux.alibaba.com>]

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