Re: [PATCH 2/4] mm/sparse: skip no-map memblock check when fill_subsection_map

[David Hildenbrand <david@redhat.com>]

On 11.07.22 14:24, Li Chen wrote:
> From: Li Chen <lchen@ambarella.com>
> 
> When mhp use sparse_add_section, don't check no-map region,
> so that to allow no-map reserved memory to get struct page
> support.
> 
> Signed-off-by: Li Chen <lchen@ambarella.com>
> Change-Id: I0d2673cec1b66adf695251037a00c240976b226f
> ---
>  mm/sparse.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 120bc8ea5293..a29cd1e7014f 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -690,7 +690,9 @@ static int fill_subsection_map(unsigned long pfn, unsigned long nr_pages)
>  
>  	if (bitmap_empty(map, SUBSECTIONS_PER_SECTION))
>  		rc = -EINVAL;
> -	else if (bitmap_intersects(map, subsection_map, SUBSECTIONS_PER_SECTION))
> +	else if (memblock_is_map_memory(PFN_PHYS(pfn)) &&
> +		 bitmap_intersects(map, subsection_map,
> +				   SUBSECTIONS_PER_SECTION))
>  		rc = -EEXIST;
>  	else
>  		bitmap_or(subsection_map, map, subsection_map,

I'm not sure I follow completely what you are trying to achieve. But if
you have to add memblock hacks into mm/sparse.c you're most probably
doing something wrong.

Please explain why that change is necessary, and why it is safe.

If the subsection map already spans memory (iow, subsection map is set)
you intend to add, then something already added memory in that range?

-- 
Thanks,

David / dhildenb