linux-kernel - Re: [PATCH 1/1] arm64: make section size configurable for memory hotplug

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Message-ID: <d0e627fd-390f-5915-c218-e2137aef3eb4@redhat.com>
Date:   Thu, 7 Jan 2021 13:30:55 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Sudarshan Rajagopalan <sudaraja@...eaurora.org>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] arm64: make section size configurable for memory
 hotplug

On 06.01.21 07:11, Anshuman Khandual wrote:
> Hi Sudershan,
> 
> This patch (and the cover letter) does not copy LAKML even though the
> entire change here is arm64 specific. Please do copy all applicable
> mailing lists for a given patch.
> 
> On 1/6/21 6:58 AM, Sudarshan Rajagopalan wrote:
>> Currently on arm64, memory section size is hard-coded to 1GB.
>> Make this configurable if memory-hotplug is enabled, to support
>> more finer granularity for hotplug-able memory.
> 
> Section size has always been decided by the platform. It cannot be a
> configurable option because the user would not know the constraints
> for memory representation on the platform and besides it also cannot
> be trusted.
> 
>>
>> Signed-off-by: Sudarshan Rajagopalan <sudaraja@...eaurora.org>
>> ---
>>  arch/arm64/Kconfig                 | 11 +++++++++++
>>  arch/arm64/include/asm/sparsemem.h |  4 ++++
>>  2 files changed, 15 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 6d232837cbee..34124eee65da 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -294,6 +294,17 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
>>  config SMP
>>  	def_bool y
>>  
>> +config HOTPLUG_SIZE_BITS
>> +	int "Memory hotplug block size(29 => 512MB 30 => 1GB)"

"Memory hotplug block size" and "HOTPLUG_SIZE_BITS" is confusing. It's
the section size. Please use that terminology.

(if at all, it would have to be "minimum memory hot(un)plug
granularity", but even that is somewhat misleading)

"SECTION_SIZE_BITS"

But I agree that letting the user configure it is sub-optimal.

>> +	depends on SPARSEMEM
>> +	depends on MEMORY_HOTPLUG
>> +	range 28 30
> 
> 28 would not work for 64K pages.

@Anshuman, what's your feeling about changing this to 128 MB for 4k/16k
base pages (as we have on x86-64 right now) and 512 MB for 64k as
default for now?

(If we worry about the number of section bits in page->flags, we could
glue it to vmemmap support where that does not matter)


-- 
Thanks,

David / dhildenb