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

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Message-ID: <20200623214847.fggchont44kgmvs6@master>
Date:   Tue, 23 Jun 2020 21:48:47 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
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, david@...hat.com
Subject: Re: [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@...ux.alibaba.com>
>
>Can a user trigger this or is this a theoretical bug?
>

I don't expect to have a non-section aligned system, so this is a theoretical
bug to me.

>> ---
>>  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.
>

Like:

    /* Only depopulate sub-section memmap for non early section. */

Looks good to you?

>>  		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