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    SubjectRe: [PATCH 1/1] arm64/sparsemem: reduce SECTION_SIZE_BITS
    On Wed, Jan 20, 2021 at 09:29:13PM -0800, Sudarshan Rajagopalan wrote:
    > memory_block_size_bytes() determines the memory hotplug granularity i.e the
    > amount of memory which can be hot added or hot removed from the kernel. The
    > generic value here being MIN_MEMORY_BLOCK_SIZE (1UL << SECTION_SIZE_BITS)
    > for memory_block_size_bytes() on platforms like arm64 that does not override.
    >
    > Current SECTION_SIZE_BITS is 30 i.e 1GB which is large and a reduction here
    > increases memory hotplug granularity, thus improving its agility. A reduced
    > section size also reduces memory wastage in vmemmmap mapping for sections
    > with large memory holes. So we try to set the least section size as possible.
    >
    > A section size bits selection must follow:
    > (MAX_ORDER - 1 + PAGE_SHIFT) <= SECTION_SIZE_BITS
    >
    > CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64 and so just following it
    > would help achieve the smallest section size.
    >
    > SECTION_SIZE_BITS = (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT)
    >
    > SECTION_SIZE_BITS = 22 (11 - 1 + 12) i.e 4MB for 4K pages
    > SECTION_SIZE_BITS = 24 (11 - 1 + 14) i.e 16MB for 16K pages without THP
    > SECTION_SIZE_BITS = 25 (12 - 1 + 14) i.e 32MB for 16K pages with THP
    > SECTION_SIZE_BITS = 26 (11 - 1 + 16) i.e 64MB for 64K pages without THP
    > SECTION_SIZE_BITS = 29 (14 - 1 + 16) i.e 512MB for 64K pages with THP
    >
    > But there are other problems in reducing SECTION_SIZE_BIT. Reducing it by too
    > much would over populate /sys/devices/system/memory/ and also consume too many
    > page->flags bits in the !vmemmap case. Also section size needs to be multiple
    > of 128MB to have PMD based vmemmap mapping with CONFIG_ARM64_4K_PAGES.
    >
    > Given these constraints, lets just reduce the section size to 128MB for 4K
    > and 16K base page size configs, and to 512MB for 64K base page size config.
    >
    > Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
    > Suggested-by: Anshuman Khandual <anshuman.khandual@arm.com>
    > Suggested-by: David Hildenbrand <david@redhat.com>
    > Cc: Catalin Marinas <catalin.marinas@arm.com>
    > Cc: Will Deacon <will@kernel.org>
    > Cc: Anshuman Khandual <anshuman.khandual@arm.com>
    > Cc: David Hildenbrand <david@redhat.com>
    > Cc: Mike Rapoport <rppt@linux.ibm.com>
    > Cc: Mark Rutland <mark.rutland@arm.com>
    > Cc: Logan Gunthorpe <logang@deltatee.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Steven Price <steven.price@arm.com>
    > Cc: Suren Baghdasaryan <surenb@google.com>

    Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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