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Subject: Re: [PATCH 1/1] arm64: make section size configurable for memory
 hotplug
To:     David Hildenbrand <david@redhat.com>,
        Sudarshan Rajagopalan <sudaraja@codeaurora.org>,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <cover.1609895500.git.sudaraja@codeaurora.org>
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 <055b0aca-af60-12ad-cd68-e15440ade64b@arm.com>
 <d0e627fd-390f-5915-c218-e2137aef3eb4@redhat.com>
 <3ae8c16d-50c4-c6cc-62b8-922cfc308c95@arm.com>
 <7939710a-5d03-de2b-73b2-bca472de431a@redhat.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <5138b97e-41f7-11c3-9a28-7fb2e2f5c387@arm.com>
Date:   Mon, 11 Jan 2021 09:47:56 +0530
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On 1/8/21 9:00 PM, David Hildenbrand wrote:
>> To summarize, the section size bits for each base page size config
>> should always
>>
>> a. Avoid (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS
> 
> Pageblocks must also always fall completely into a section.
> 
>>
>> b. Provide minimum possible section size for a given base page config to
>>    have increased agility during memory hotplug operations and reduced
>>    vmemmap wastage for sections with holes.
> 
> OTOH, making the section size too small (e.g., 16MB) creates way to many
> memory block devices in /sys/devices/system/memory/, and might consume
> too many page->flags bits in the !vmemmap case.
> 
> For bigger setups, we might, similar to x86-64 (e.g., >= 64 GiB),
> determine the memory_block_size_bytes() at runtime (spanning multiple
> sections then), once it becomes relevant.
> 
>>
>> c. Allow 4K base page configs to have PMD based vmemmap mappings
> 
> Agreed.
> 
>>
>> Because CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64 platform,
>> the following would always avoid the condition (a)
>>
>> SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT)
>>
>> 			- 22 (11 - 1 + 12) for 4K pages
>> 			- 24 (11 - 1 + 14) for 16K pages without THP
>> 			- 25 (12 - 1 + 14) for 16K pages with THP
>> 			- 26 (11 - 1 + 16) for 64K pages without THP
>> 			- 29 (14 - 1 + 16) for 64K pages with THP
>>
>> Apart from overriding 4K base page size config to have 27 as section size
>> bits, should not all other values be okay here ? But then wondering what
>> benefit 128MB (27 bits) section size for 16K config would have ? OR the
>> objective here is to match 16K page size config with default x86-64.
> 
> We don't want to have sections that are too small. We don't want to have
> sections that are too big :)
> 
> Not sure if we really want to allow setting e.g., a section size of 4
> MB. That's just going to hurt. IMHO, something in the range of 64..256
> MB is quite a good choice, where possible.
> 
>>
>>>
>>> (If we worry about the number of section bits in page->flags, we could
>>> glue it to vmemmap support where that does not matter)
>>
>> Could you please elaborate ? Could smaller section size bits numbers like
>> 22 or 24 create problems in page->flags without changing other parameters
>> like NR_CPUS or NODES_SHIFT ? A quick test with 64K base page without THP
> 
> Yes, in the !vmemmap case, we have to store the section_nr in there.
> IIRC, it's less of an issue with section sizes like 128 MB.
> 
>> i.e 26 bits in section size, fails to boot.
> 
> 26 bits would mean 64 MB, no? Not sure if that's possible even without
> THP (MAX_ORDER - 1, pageblock_order ...) on 64k pages. I'd assume 512 MB
> is the lowest we can go. I'd assume this would crash :)
> 
>>
>> As you have suggested, probably constant defaults (128MB for 4K/16K, 512MB
>> for 64K) might be better than depending on the CONFIG_FORCE_MAX_ZONEORDER,
>> at least for now.
> 
> That's also what I would prefer, keeping it simple.

Okay sure, will send a RFC to begin with.