linux-kernel - Re: [RFC PATCH v2] page_alloc: allow migration of smaller hugepages during contig

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Message-ID: <aPevZplPtDUPZFUn@gourry-fedora-PF4VCD3F>
Date: Tue, 21 Oct 2025 12:05:58 -0400
From: Gregory Price <gourry@...rry.net>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	akpm@...ux-foundation.org, vbabka@...e.cz, surenb@...gle.com,
	mhocko@...e.com, jackmanb@...gle.com, hannes@...xchg.org,
	ziy@...dia.com
Subject: Re: [RFC PATCH v2] page_alloc: allow migration of smaller hugepages
 during contig_alloc.

On Tue, Oct 21, 2025 at 11:30:48AM +0200, David Hildenbrand wrote:
> > Compaction `isolate_migrate_pages_block()` already expects requests
> 
> Please, let's not talk about "compaction" here, it's just confusing to talk
> about compaction for something that is not compaction but uses some
> primitives (because not properly separated yet)
> 
> Just say "isolate_migrate_pages_block() already expects ..."
>

will do

> > +			/* Don't consider moving same size/larger pages */
> > +			page = compound_head(page);
> > +			order = compound_order(page);
> > +			if ((order >= MAX_PAGE_ORDER) ||
> > +			    (nr_pages < (1 << order)))
> > +				return false;
> 
> This is roughly what we do in pageblock_skip_persistent(), just with a
> hardcoded pageblock size.
> 
> I'm not sure about the MAX_PAGE_ORDER check, though. If an arch supports two
> hugetlb sizes that exceed MAX_PAGE_ORDER, it would not work as expected.
> 
> Doesn't arm64 support that with cont-PMD vs. PUD hugetlb folios?
> MAX_FOLIO_ORDER would be better.

Will adjust to MAX_FOLIO_ORDER, makes sense.  Missed that.  Will take a
look at the pageblock code, I think I was looking at the compaction and
some other code for inspiration here.

~Gregory