Re: [PATCH v3] mm: Add CONFIG_PAGE_BLOCK_ORDER to select page block order

[Vlastimil Babka <vbabka@suse.cz>]

On 5/6/25 09:01, Andrew Morton wrote:
> On Mon,  5 May 2025 17:22:58 -0700 Juan Yescas <jyescas@google.com> wrote:
> 
>> Problem: On large page size configurations (16KiB, 64KiB), the CMA
>> alignment requirement (CMA_MIN_ALIGNMENT_BYTES) increases considerably,
>> and this causes the CMA reservations to be larger than necessary.
>> This means that system will have less available MIGRATE_UNMOVABLE and
>> MIGRATE_RECLAIMABLE page blocks since MIGRATE_CMA can't fallback to them.
>> 
>> The CMA_MIN_ALIGNMENT_BYTES increases because it depends on
>> MAX_PAGE_ORDER which depends on ARCH_FORCE_MAX_ORDER. The value of
>> ARCH_FORCE_MAX_ORDER increases on 16k and 64k kernels.
>> 
>> ...
>>
>> +config PAGE_BLOCK_ORDER
>> +	int "Page Block Order"
>> +	range 1 10 if !ARCH_FORCE_MAX_ORDER
>> +	default 10 if !ARCH_FORCE_MAX_ORDER
>> +	range 1 ARCH_FORCE_MAX_ORDER if ARCH_FORCE_MAX_ORDER
>> +	default ARCH_FORCE_MAX_ORDER if ARCH_FORCE_MAX_ORDER
> 
> Do we really need to do this arithmetic within Kconfig?  Would it be
> cleaner to do this at runtime, presumably when calculating
> pageblock_order?

AFAIK pageblock_order is compile-time constant. Making this a boot parameter
was proposed in v1 but explained as not useful. That explanation could be
added in the changelog?