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Date: Tue, 23 Jan 2024 15:01:38 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
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	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Dinh Nguyen <dinguyen@kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	"Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
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Subject: Re: [PATCH v1 01/11] arm/pgtable: define PFN_PTE_SHIFT on arm and
 arm64
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 <20240122194200.381241-2-david@redhat.com>
 <fdaeb9a5-d890-499a-92c8-d171df43ad01@arm.com>
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On Tue, Jan 23, 2024 at 10:34:21AM +0000, Ryan Roberts wrote:
> > +#define PFN_PTE_SHIFT		PAGE_SHIFT
> 
> I think this is buggy. And so is the arm64 implementation of set_ptes(). It
> works fine for 48-bit output address, but for 52-bit OAs, the high bits are not
> kept contigously, so if you happen to be setting a mapping for which the
> physical memory block straddles bit 48, this won't work.

I'd like to see the folio allocation that can straddle bit 48 ...

agreed, it's not workable _in general_, but specifically for a memory
allocation from a power-of-two allocator, you'd have to do a 49-bit
allocation (half a petabyte) to care.