linux-kernel - Re: [PATCH] mm: Add ARCH_FORCE_PAGE_BLOCK

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Message-ID: <20250502113733.GA29622@willie-the-truck>
Date: Fri, 2 May 2025 12:37:34 +0100
From: Will Deacon <will@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Juan Yescas <jyescas@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, tjmercier@...gle.com, isaacmanjarres@...gle.com,
	surenb@...gle.com, kaleshsingh@...gle.com,
	Vlastimil Babka <vbabka@...e.cz>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	Mike Rapoport <rppt@...nel.org>, Zi Yan <ziy@...dia.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH] mm: Add ARCH_FORCE_PAGE_BLOCK_ORDER to select page block
 order

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