Re: [PATCH 3/7] mm/sparse.c: only use subsection map in VMEMMAP case

[Dan Williams]

On Sun, Feb 9, 2020 at 2:48 AM Baoquan He <bhe@xxxxxxxxxx> wrote:
>
> Currently, subsection map is used when SPARSEMEM is enabled, including
> VMEMMAP case and !VMEMMAP case. However, subsection hotplug is not
> supported at all in SPARSEMEM|!VMEMMAP case, subsection map is unnecessary
> and misleading. Let's adjust code to only allow subsection map being
> used in SPARSEMEM|VMEMMAP case.
>
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> ---
>  include/linux/mmzone.h |   2 +
>  mm/sparse.c            | 231 ++++++++++++++++++++++-------------------
>  2 files changed, 124 insertions(+), 109 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 462f6873905a..fc0de3a9a51e 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1185,7 +1185,9 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
>  #define SUBSECTION_ALIGN_DOWN(pfn) ((pfn) & PAGE_SUBSECTION_MASK)
>
>  struct mem_section_usage {
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
>         DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
> +#endif

This was done deliberately so that the SPARSEMEM_VMEMMAP=n case ran as
a subset of the SPARSEMEM_VMEMMAP=y case.

The diffstat does not seem to agree that this is any clearer:

    124 insertions(+), 109 deletions(-)