linux-kernel - Re: [RFC PATCH v1 57/57] arm64: Enable boot-time page size selection

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Message-ID: <daecf1d1-04c7-4513-86db-397c2ef6f768@arm.com>
Date: Wed, 16 Oct 2024 09:14:41 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Zi Yan <ziy@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 David Hildenbrand <david@...hat.com>, Greg Marsden
 <greg.marsden@...cle.com>, Ivan Ivanov <ivan.ivanov@...e.com>,
 Kalesh Singh <kaleshsingh@...gle.com>, Marc Zyngier <maz@...nel.org>,
 Mark Rutland <mark.rutland@....com>, Matthias Brugger <mbrugger@...e.com>,
 Miroslav Benes <mbenes@...e.cz>, Oliver Upton <oliver.upton@...ux.dev>,
 Will Deacon <will@...nel.org>, kvmarm@...ts.linux.dev,
 linux-arm-kernel@...ts.infradead.org, linux-efi@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 57/57] arm64: Enable boot-time page size selection

On 15/10/2024 18:42, Zi Yan wrote:
> On 14 Oct 2024, at 6:59, Ryan Roberts wrote:
> 
>> Introduce a new Kconfig, ARM64_BOOT_TIME_PAGE_SIZE, which can be
>> selected instead of a page size. When selected, the resulting kernel's
>> page size can be configured at boot via the command line.
>>
>> For now, boot-time page size kernels are limited to 48-bit VA, since
>> more work is required to support LPA2. Additionally MMAP_RND_BITS and
>> SECTION_SIZE_BITS are configured for the worst case (64K pages). Future
>> work could be implemented to be able to configure these at boot time for
>> optimial page size-specific values.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
> 
> <snip>
> 
>>
>> @@ -1588,9 +1601,10 @@ config XEN
>>  # 4K  |       27          |      12      |       15             |         10              |
>>  # 16K |       27          |      14      |       13             |         11              |
>>  # 64K |       29          |      16      |       13             |         13              |
>> +# BOOT|       29          |    16 (max)  |       13             |         13              |
>>  config ARCH_FORCE_MAX_ORDER
>>  	int
>> -	default "13" if ARM64_64K_PAGES
>> +	default "13" if ARM64_64K_PAGES || ARM64_BOOT_TIME_PAGE_SIZE
>>  	default "11" if ARM64_16K_PAGES
>>  	default "10"
>>  	help
> 
> So boot-time page size kernel always has the highest MAX_PAGE_ORDER, which
> means the section size increases for 4KB and 16KB page sizes. Any downside
> for this?

I guess there is some cost to the buddy when MAX_PAGE_ORDER is larger than it
needs to be - I expect you can explain those details much better than I can. I'm
just setting it to the worst case for now as it was the easiest solution for the
initial series.

> 
> Is there any plan (not in this patchset) to support boot-time MAX_PAGE_ORDER
> to keep section size the same?

Yes absolutely. I should have documented MAX_PAGE_ORDER in the commit log along
with the comments for MMAP_RND_BITS and SECTION_SIZE_BITS - that was an
oversight and I'll fix it in the next version. I plan to look at making all 3
values boot-time configurable in future (although I have no idea at this point
how involved that will be).

Thanks,
Ryan

> 
> Best Regards,
> Yan, Zi