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

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Date:   Tue, 23 Jun 2020 14:44:02 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        akpm@...ux-foundation.org, osalvador@...e.de,
        dan.j.williams@...el.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section

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.

-- 
Thanks,

David / dhildenb