linux-kernel - Re: [PATCH 2/2] mm: convert do_set

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Message-ID: <aBytpMWqzfjgIy4U@casper.infradead.org>
Date: Thu, 8 May 2025 14:12:04 +0100
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
	akpm@...ux-foundation.org, hannes@...xchg.org,
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
	npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
	vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: convert do_set_pmd() to take a folio

On Thu, May 08, 2025 at 09:36:02AM +0200, David Hildenbrand wrote:
> > > Which raises an interesting question: I assume in the future, when we have a 4 MiB folio on x86-64 that is *misaligned* in VA space regarding PMDs (e.g., aligned to 1 MiB but not 2 MiB), we could still allow to use a PMD for the middle part.
> > 
> > It might not be possible if the folio comes from buddy allocator due to how
> > buddy allocator merges a PFN with its buddy (see __find_buddy_pfn() in mm/internal.h).
> > A 4MB folio will always be two 2MB-aligned parts. In addition, VA and PA need
> > to have the same lower 9+12 bits for a PMD mapping. So PMD mappings for
> > a 4MB folio would always be two PMDs. Let me know if I miss anything.
> 
> PA is clear. But is mis-alignment in VA space impossible on all
> architectures? I certainly remember it being impossible on x86-64 and s390x
> (remaining PMD entry bits used for something else).

Yeah, that's not possible; the PMD is always PMD-aligned.