Re: [PATCH 1/1] arm64: reduce section size for sparsemem

[Sudarshan Rajagopalan <sudaraja@codeaurora.org>]

On 2021-01-20 09:49, Will Deacon wrote:
> On Fri, Jan 08, 2021 at 03:16:00PM -0800, Sudarshan Rajagopalan wrote:
>> Reducing the section size helps reduce wastage of reserved memory
>> for huge memory holes in sparsemem model. But having a much smaller
>> section size bits could break PMD mappings for vmemmap and wouldn't
>> accomodate the highest order page for certain page size granule 
>> configs.
>> It is determined that SECTION_SIZE_BITS of 27 (128MB) could be ideal
>> default value for 4K_PAGES that gives least section size without 
>> breaking
>> PMD based vmemmap mappings. For simplicity, 16K_PAGES could follow the
>> same as 4K_PAGES. And the least SECTION_SIZE_BITS for 64K_PAGES is 29
>> that could accomodate MAX_ORDER.
>> 
>> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Cc: Mike Rapoport <rppt@linux.ibm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Suren Baghdasaryan <surenb@google.com>
>> ---
>>  arch/arm64/include/asm/sparsemem.h | 10 ++++++++--
>>  1 file changed, 8 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/arm64/include/asm/sparsemem.h 
>> b/arch/arm64/include/asm/sparsemem.h
>> index 1f43fcc79738..ff08ff6b677c 100644
>> --- a/arch/arm64/include/asm/sparsemem.h
>> +++ b/arch/arm64/include/asm/sparsemem.h
>> @@ -7,7 +7,13 @@
>> 
>>  #ifdef CONFIG_SPARSEMEM
>>  #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
>> -#define SECTION_SIZE_BITS	30
>> -#endif
>> +
>> +#if defined(CONFIG_ARM64_4K_PAGES) || defined(CONFIG_ARM64_16K_PAGES)
>> +#define SECTION_SIZE_BITS 27
>> +#else
>> +#define SECTION_SIZE_BITS 29
>> +#endif /* CONFIG_ARM64_4K_PAGES || CONFIG_ARM64_16K_PAGES */
>> +
>> +#endif /* CONFIG_SPARSEMEM*/
> 
> Please can you repost this in light of the comments from Anshuman?
> 
> Thanks,
> 
> Will

Sure Will. We were held up with some other critical tasks.. will repost 
the patch by EOD after addressing Anshuman's comments.

-- 
Sudarshan

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