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

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Date:   Tue, 23 Jun 2020 21:02:09 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        akpm@...ux-foundation.org, osalvador@...e.de,
        dan.j.williams@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section

On Tue, Jun 23, 2020 at 02:44:02PM +0200, David Hildenbrand wrote:
>On 23.06.20 11:42, 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@...ux.alibaba.com>
>> ---
>>  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)
>>  		depopulate_section_memmap(pfn, nr_pages, altmap);
>> +	else if (memmap)
>> +		free_map_bootmem(memmap);
>>  
>>  	if (empty)
>>  		ms->section_mem_map = (unsigned long)NULL;
>> 
>
>Agreed, that's what pfn_valid() and section_activate() expect.
>
>"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." - this
>is the case when hot-adding sub-sections into partially populated early
>sections, and has to be the case when re-hot-adding after hot-removing.
>
>Acked-by: David Hildenbrand <david@...hat.com>
>
>
>I am also not convinced that the complicated sparse_decode_mem_map()
>handling in that function is required - ms->section_mem_map &
>SECTION_MAP_MASK is sufficient for this use case of removing the memmap
>of a full early section once empty.
>

You mean remove this line?

    	memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);

Then what to passed to free_map_bootmem() ?

>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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