Re: [PATCH 1/3] mm/sparsemem: adjust memmap only for SPARSEMEM_VMEMMAP

[Dan Williams]

On Thu, Feb 6, 2020 at 3:17 PM Wei Yang <richardw.yang@xxxxxxxxxxxxxxx> wrote:
>
> Only when SPARSEMEM_VMEMMAP is set, memmap returned from
> section_activate() points to sub-section page struct. Otherwise, memmap
> already points to the whole section page struct.
>
> This means only for SPARSEMEM_VMEMMAP, we need to adjust memmap for
> sub-section case.
>
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
> CC: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  mm/sparse.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 586d85662978..b5da121bdd6e 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -886,7 +886,8 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
>         section_mark_present(ms);
>
>         /* Align memmap to section boundary in the subsection case */
> -       if (section_nr_to_pfn(section_nr) != start_pfn)
> +       if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
> +               section_nr_to_pfn(section_nr) != start_pfn)

Aren't we assured that start_pfn is always section aligned in the
SPARSEMEM case? That's the role of check_pfn_span(). Does the change
have a runtime impact or is this just theoretical?