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Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section
To:     Wei Yang <richard.weiyang@linux.alibaba.com>
Cc:     akpm@linux-foundation.org, osalvador@suse.de,
        dan.j.williams@intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20200623094258.6705-1-richard.weiyang@linux.alibaba.com>
 <414b4cbf-02f5-0e90-df92-c0889e9ad65b@redhat.com>
 <20200623130209.GA8347@L-31X9LVDL-1304.local>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <b4ab4787-543c-ca45-45e8-03eafa2b6105@redhat.com>
Date:   Tue, 23 Jun 2020 15:16:23 +0200
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Sender: linux-kernel-owner@vger.kernel.org
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On 23.06.20 15:02, Wei Yang wrote:
> 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@linux.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@redhat.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() ?

Never mind, I misread something,  sparse_decode_mem_map() is indeed
necessary.


-- 
Thanks,

David / dhildenb