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From:   David Hildenbrand <david@redhat.com>
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Subject: Re: [PATCH 3/7] mm/sparse.c: only use subsection map in VMEMMAP case
Date:   Wed, 12 Feb 2020 10:39:56 +0100
Message-Id: <B23A05D9-40E2-4862-979D-C6DA69DDDC80@redhat.com>
References: <CAPcyv4hh5PmF8qU+p7Q903PhX+ho9yHMzLFncmh6psW5YOLU_w@mail.gmail.com>
Cc:     Baoquan He <bhe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Wei Yang <richardw.yang@linux.intel.com>,
        David Hildenbrand <david@redhat.com>
In-Reply-To: <CAPcyv4hh5PmF8qU+p7Q903PhX+ho9yHMzLFncmh6psW5YOLU_w@mail.gmail.com>
To:     Dan Williams <dan.j.williams@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> Am 11.02.2020 um 21:15 schrieb Dan Williams <dan.j.williams@intel.com>:
>=20
> =EF=BB=BFOn Sun, Feb 9, 2020 at 2:48 AM Baoquan He <bhe@redhat.com> wrote:=

>>=20
>> 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 unnecessar=
y
>> and misleading. Let's adjust code to only allow subsection map being
>> used in SPARSEMEM|VMEMMAP case.
>>=20
>> Signed-off-by: Baoquan He <bhe@redhat.com>
>> ---
>> include/linux/mmzone.h |   2 +
>> mm/sparse.c            | 231 ++++++++++++++++++++++-------------------
>> 2 files changed, 124 insertions(+), 109 deletions(-)
>>=20
>> 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(unsig=
ned long sec)
>> #define SUBSECTION_ALIGN_DOWN(pfn) ((pfn) & PAGE_SUBSECTION_MASK)
>>=20
>> struct mem_section_usage {
>> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
>>        DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
>> +#endif
>=20
> This was done deliberately so that the SPARSEMEM_VMEMMAP=3Dn case ran as
> a subset of the SPARSEMEM_VMEMMAP=3Dy case.
>=20
> The diffstat does not seem to agree that this is any clearer:
>=20
>    124 insertions(+), 109 deletions(-)
>=20

I don=E2=80=98t see a reason to work with subsections (+store them) if subse=
ctions are not supported.

I do welcome this cleanup. Diffstats don=E2=80=98t tell the whole story.=