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

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Date:   Wed, 12 Feb 2020 07:48:15 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Baoquan He <bhe@...hat.com>,
        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>
Subject: Re: [PATCH 3/7] mm/sparse.c: only use subsection map in VMEMMAP case

On Wed, Feb 12, 2020 at 1:40 AM David Hildenbrand <david@...hat.com> wrote:
>
>
>
> > Am 11.02.2020 um 21:15 schrieb Dan Williams <dan.j.williams@...el.com>:
> >
> > 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(-)
> >
>
> I don‘t see a reason to work with subsections (+store them) if subsections are not supported.
>
> I do welcome this cleanup. Diffstats don‘t tell the whole story.

I'll take a look at the final result and see if my opinion changes,
but I just wanted to clarify upfront that making sparsemem run some of
the subsection logic was deliberate.