[PATCH] mm: Add ARCH_FORCE_PAGE_BLOCK_ORDER to select page block order

[Will Deacon]

On Thu, May 01, 2025 at 07:38:13PM +0100, Matthew Wilcox wrote:
> On Wed, Apr 30, 2025 at 10:25:11PM -0700, Juan Yescas 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.
> 
> Sure, but why would any architecture *NOT* want to set this?
> This seems like you're making each architecture bump into the problem
> by itself, when the real problem is that the CMA people never thought
> about this and should have come up with better defaults.

Yes, I agree. It would be nice if arm64 wasn't the odd duck here. You'd
think Power and Risc-V would benefit from similar treatement, if nothing
else.

Will