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Date:   Fri, 8 Jan 2021 16:30:31 +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

> 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.

-- 
Thanks,

David / dhildenb

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