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

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Message-ID: <CAPcyv4hh5PmF8qU+p7Q903PhX+ho9yHMzLFncmh6psW5YOLU_w@mail.gmail.com>
Date:   Tue, 11 Feb 2020 12:14:54 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Yang <richardw.yang@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 3/7] mm/sparse.c: only use subsection map in VMEMMAP case

On Sun, Feb 9, 2020 at 2:48 AM Baoquan He <bhe@...hat.com> 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@...hat.com>
> ---
>  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(-)