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 10:39:56 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.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>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 3/7] mm/sparse.c: only use subsection map in VMEMMAP case



> 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.