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 19:20:00 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...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>
Subject: Re: [PATCH 3/7] mm/sparse.c: only use subsection map in VMEMMAP case

On 02/12/20 at 10:39am, David Hildenbrand 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.

Thanks for checking this, Dan.

Taking away the subsection part, won't affect the classic sparse being a
subset of VMEMMAP case, I would say.


> > 
> > 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.

Thanks for clarifying this, David, I agree.

If applying the patch, it should be easier to observe that the code
is simpler to follow, at least won't be confusing on subsection handling
part.